feat(pilot): emit per-review Langfuse traces

Ship token spend, latency and equivalent cost for every review to the
self-hosted Langfuse so per-model behaviour is queryable as a trend rather
than one PR comment at a time.

langfuse_trace.py is stdlib-only and emits via the public ingestion API.
Traces split into `ollama` and `claude` environments keyed off the bare model
name, not the provider: both paths go through the same headroom proxy, so the
provider prefix says nothing about which spend story a review belongs to. The
pilot's own path bills $0, so the reported cost is the equivalent price from
cost_model.PRICES.

ai_review.py calls _emit_langfuse on both token-spending exit paths (the
normal post and the salvage path). Import and emission are wrapped in a
blanket except: with no LANGFUSE_HOST or key pair the whole thing is a silent
no-op, and a telemetry failure must never fail a review.

These files were previously deployed only by way of the image build's
`COPY . /app`, so a clean checkout would have silently dropped tracing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

pilot/README-langfuse.md

@@ -0,0 +1,122 @@
+# pragent → Langfuse
+
+Every review the pilot runs ships one **trace** to a self-hosted Langfuse. The
+review body already prints a usage table, but that table lives and dies inside
+one Gitea PR. Langfuse is where the same numbers become a trend: tokens per
+review, latency per model, equivalent cost per repo, and how those move when
+the model or the tiering changes.
+
+## The ollama / claude split
+
+Both paths route through the same headroom proxy, so the provider prefix does
+not distinguish them — `headroom/claude-sonnet-5` is Claude spend,
+`headroom/glm-5.2:cloud` is not. The split is keyed off the **bare model name**
+and lands on the trace's `environment`:
+
+| resolved model              | environment |
+| --------------------------- | ----------- |
+| `headroom/claude-sonnet-5`  | `claude`    |
+| `claude-opus-5`             | `claude`    |
+| `headroom/glm-5.2:cloud`    | `ollama`    |
+| `headroom/MiniMax-M2.7`     | `ollama`    |
+| `vllm-qwen38/qwen3.8-27b`   | `ollama`    |
+
+Langfuse takes an environment selector on every dashboard, filter and cost
+breakdown, so the two spend stories stay separate inside one project — one key
+pair to rotate instead of two. Tags carry the finer cut:
+`provider:headroom`, `model:<bare>`, `engine:opencode`, `repo:<owner/name>`,
+`lens:<id>` per fan-out lens.
+
+To split into two *projects* later, point `LANGFUSE_PUBLIC_KEY` /
+`LANGFUSE_SECRET_KEY` at the second project on whichever deployment runs the
+Claude path. Nothing in the code needs to change.
+
+## What a trace carries
+
+- **trace** `pr-review` — `sessionId` = `owner/repo#index`, so every push to one
+  PR groups together. Input is the PR identity; output is the summary + finding
+  count; metadata carries steps, duration, severity counts and the provider's
+  own reported cost.
+- **generation** `opencode-review` — `model`, `usageDetails`, `costDetails`.
+
+`usageDetails.input` is the **uncached** input. opencode reports `cache_read`
+*inside* `input`, and Langfuse sums the keys it is given, so passing both
+verbatim would bill the resent prefix twice.
+
+### How cost is priced
+
+Langfuse has no price table of its own here — we compute the number and ship it
+as `costDetails.total`, so what Langfuse charts is exactly what
+`cost_model.PRICES` says.
+
+A model that genuinely bills (`claude-*`, `gpt-*`, `gemini-*`, `grok-*`) is
+priced **as itself**: basis `actual`.
+
+A model that costs nothing through the headroom proxy is priced against a
+**comparison target** instead: basis `equivalent:<target>`. That covers the
+models absent from `PRICES` (`MiniMax-M2.7` — which is what the webhook
+actually runs — and `glm-5.2:cloud`) as well as entries priced at all zeros
+(the self-hosted vLLM `qwen3.8-27b`). Without this the dashboard would be a
+flat $0.00 line, since the pilot's own path is free.
+
+The target follows the same precedence as the review body, so the PR and the
+dashboard never disagree:
+
+    .pr-review.json:cost_target  >  PRAGENT_PRICE_TARGET  >  claude-sonnet-5
+
+An equivalent cost is a hypothetical, not money spent, so every trace is tagged
+`cost:actual` or `cost:equivalent:<target>` and the generation metadata carries
+`cost_basis`. Filter on it before reading any cost chart as spend.
+
+If the comparison target itself is unknown, the trace ships usage with **no**
+cost block — better no number than a wrong one.
+
+Anthropic prices in `cost_model.PRICES` were fetched 2026-08-18; re-check them
+before quoting anything externally.
+
+## Configuration
+
+| env                   | meaning                                                   |
+| --------------------- | --------------------------------------------------------- |
+| `LANGFUSE_HOST`       | `http://langfuse-web.langfuse.svc.cluster.local:3000`      |
+| `LANGFUSE_PUBLIC_KEY` | `pk-lf-…`                                                  |
+| `LANGFUSE_SECRET_KEY` | `sk-lf-…`                                                  |
+| `LANGFUSE_TIMEOUT`    | seconds, default `5`                                       |
+| `LANGFUSE_DEBUG`      | `1` to log ingestion failures to stderr                    |
+
+Unset host or either key ⇒ emission is a silent no-op. That is the default, so
+a checkout without Langfuse behaves exactly as before.
+
+## Fail-open
+
+`langfuse_trace` is stdlib-only (`urllib`) and every entry point swallows its
+own exceptions; `_emit_langfuse` in `ai_review.py` wraps even the import. A
+Langfuse outage cannot fail, delay past `LANGFUSE_TIMEOUT`, or alter a review.
+
+Both token-spending exit paths emit — the normal post **and** the salvage path
+where the agent produced unparseable output. That run cost the same as a clean
+one, and is precisely the failure worth trending.
+
+## Deployment
+
+Cluster side lives outside this repo: `~/k8s/langfuse.yaml` (ClickHouse +
+web + worker, reusing the gitea postgres, gitea valkey and minio),
+`~/k8s/oauth2-proxy-langfuse.yaml` (the Logto gate), and
+`~/k8s/langfuse-setup.sh`, which provisions the database, the bucket, the
+secrets, and wires `pragent-webhook` with the three env vars above.
+
+The UI is at **https://langfuse.marcospaulo.dev.br**:
+
+    browser -> Caddy (VPS, TLS, DNS-01) -> tailscale
+            -> 100.74.17.70:30361 -> oauth2-proxy (Logto, email allowlist)
+            -> langfuse-web (ClusterIP)
+
+Logto sits at *both* layers off one app (`langfuse`, two redirect URIs): the
+proxy gates the domain, and Langfuse's own NextAuth uses the same Logto as a
+custom OIDC provider, so the inner login is a silent redirect rather than a
+second password.
+
+pragent does **not** go through any of that. It posts to
+`langfuse-web.langfuse.svc.cluster.local:3000` from inside the cluster, on
+API-key auth — putting ingestion behind an interactive SSO gate would break it
+on the first review.