feat(review): ADDITIONAL_CONTEXT_URL — repo-provided static context cached per review

Long agent loops re-send the brief prefix on every step; cheap reusable
knowledge (architecture summary, module map, conventions, glossary) belongs
in a versioned file the maintainers control so the agent doesn't re-derive
it from the source tree on every PR. Two wiring paths, merged (env first):

* env var PRAGENT_ADDITIONAL_CONTEXT_URL — comma-separated, deployment-wide
* .pr-review.json:additional_context_urls — list[str], read from the PR's
  base branch (same trust boundary as the rest of the file)

Implementation:
* _parse_additional_context_env splits/dedupes/trims.
* _resolve_additional_context_urls(config) merges env (first) + config
  (then, skipping env-dupes); caps at 8.
* fetch_additional_context(urls) fetches each URL with urllib (5s timeout,
  http/https only — file://, javascript:, ftp:// rejected defensively),
  caches by URL in a module-level dict for the pod lifetime, truncates
  per-URL to 4k chars + total to 16k chars, best-effort (network errors
  are logged and skipped — never aborts the review).
* Result injected into build_user_prompt under "## Repo-provided context"
  between repo config and prior reviews. In the opencode engine it lands
  in .pragent/brief.md under its own section. The brief explicitly labels
  each block's CONTENT as untrusted (same as PR description) — section
  heading is trustworthy, body isn't.
* parse_repo_config accepts the field, caps at 8 entries, drops
  non-strings and empty strings.

Docs: pilot/README-webhook.md "Repo-provided static context" section —
env var + JSON example + Nexus raw-hosted recipe.

Tests: 14 new (208 total), covering env merging + dedup, scheme rejection,
per-URL cap, total cap, caching by URL, brief injection. All mock urllib
with a context-manager stand-in (no real network).

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

pilot/README-webhook.md

@@ -95,6 +95,63 @@ Without `AI-USAGE` (regression): no usage section, no 🪙 lines — behaviour
 identical to before the feature. The usage section is part of the review body,
 so it's covered by the existing sha-marker dedupe.
 
+## Repo-provided static context (`ADDITIONAL_CONTEXT_URL`)
+
+Long agent loops resend the brief prefix on every step; the cheap reusable
+knowledge — architecture summary, module map, conventions, glossary, past
+incident write-ups — lives in a versioned file the maintainers control, so
+the agent doesn't have to re-read the source tree to rediscover it on every
+PR. Two ways to wire it up:
+
+**Env var** (Deployment-wide, useful for shared house docs):
+
+```bash
+PRAGENT_ADDITIONAL_CONTEXT_URL="https://nexus.example/raw/architecture.md,https://nexus.example/raw/glossary.md"
+# comma-separated, trimmed, deduped; ≤ 8 URLs total
+```
+
+**Per-repo `.pr-review.json`** (read from the PR's base branch — same trust
+boundary as the rest of `.pr-review.json`):
+
+```json
+{
+  "additional_context_urls": [
+    "https://nexus.example/repository/raw-hosted/architecture.md",
+    "https://nexus.example/repository/raw-hosted/conventions.md"
+  ]
+}
+```
+
+The two are merged: env first (in declared order), then config entries that
+aren't already in env. The first 8 win.
+
+**Behaviour**:
+
+- Fetched **once per review**, cached by URL for the lifetime of the pod.
+- **http/https only** — `file://`, `javascript:`, `ftp://`, anything else is
+  silently dropped.
+- 5 s timeout per URL.
+- Per-URL truncated to **4 000 chars**, total to **16 000 chars**, then
+  `…[truncated]` is appended and the next URL is skipped.
+- Best-effort: a network error or non-200 is logged to stderr and skipped —
+  never aborts the review.
+- Rendered into the brief under **"Repo-provided context"**, between the
+  repo config and prior reviews. The brief explicitly labels the *content*
+  of each block as untrusted author-controlled data (same as the PR
+  description), so the agent knows to ground findings against it but not
+  take instructions from it.
+
+**Self-hosted example (Nexus `raw-hosted`)**:
+
+```bash
+# Upload a doc to Nexus raw-hosted (anonymous read for in-cluster pods).
+curl -u techspark -X PUT \
+  --data-binary @architecture.md \
+  https://nexus.example/repository/raw-hosted/architecture.md
+# Then reference it from .pr-review.json (above). Cache control is
+# browser-style: anonymous read = max-age from response headers.
+```
+
 ## Webhook fires on any PR update (except `closed`)
 
 The receiver uses a **denylist**, not an allowlist: it reviews on every
@@ -364,7 +421,9 @@ Env on the Deployment: `PRAGENT_ENGINE`, `OPENCODE_MODEL`,
 `OPENCODE_EXPERIMENTAL_LSP_TOOL`, `PRAGENT_FACTORY_DIR`, `PRAGENT_OPENCODE_BIN`,
 `PRAGENT_WORK_ROOT`, `PRAGENT_REVIEW_TIMEOUT`, `GITEA_API`, `OLLAMA_URL`,
 `OLLAMA_MODEL`, `OLLAMA_MAX_TOKENS`, `DIFF_MAX_CHARS`,
-`PRAGENT_MAX_CONCURRENT_REVIEWS`, `PRAGENT_MAX_BODY_BYTES` are literals;
+`PRAGENT_ADDITIONAL_CONTEXT_URL` (optional, see "Repo-provided static
+context" above), `PRAGENT_MAX_CONCURRENT_REVIEWS`, `PRAGENT_MAX_BODY_BYTES`
+are literals;
 `WEBHOOK_SECRET` + `PRAGENT_BOT_TOKEN` come from the Secret. The image now runs
 as uid 10001 — add `securityContext: {runAsNonRoot: true, runAsUser: 10001,
 fsGroup: 10001}` to the pod spec so the `/tmp/pragent-work` emptyDir is writable.