test(budget): add DesignDocs tests; replace PLAN-137 with clean design doc

- budget/budget_test.go: add TestFit_DesignDocsInSystemPrompt,
  TestFit_DesignDocsTrimmedBeforeFileContext, TestFit_DesignDocsEmptyNoHeading
  to cover the new DesignDocs section through Fit() and buildResult()
- Remove PLAN-137.md (contained raw thinking stream, not suitable as repo doc)
- Add docs/DESIGN-137-doc-map.md with clean architectural decision record

docs/DESIGN-137-doc-map.md

@@ -0,0 +1,82 @@
+# Design: doc-map input for path-scoped design doc injection (Issue #137)
+
+## Problem
+
+review-bot can inject context via `patterns-repo` (external VCS repos) and `conventions-file`
+(a single file from the reviewed repo). There is no mechanism to inject local repo documentation
+files scoped to the paths changed in a PR.
+
+First consumer: `grgl/gargoyle#778` needs a "doc adherence" reviewer that checks code against the
+module's governing design doc, without injecting every doc in the tree.
+
+## Approach
+
+### New: `doc-map` input
+
+A `.review-bot/doc-map.yml` config file in the reviewed repo maps source path globs to governing
+design docs. review-bot reads the map, intersects it with changed PR paths, and injects only the
+relevant docs into the system prompt.
+
+### Config format
+
+```yaml
+mappings:
+  - paths:
+      - "lib/gargoyle/engine/signal_risk/**"
+    docs:
+      - docs/domain/contexts/risk/risk-controls.md
+  - paths:
+      - "lib/gargoyle/trading/**"
+    docs:
+      - docs/domain/contexts/trading/
+```
+
+- `paths` — glob patterns (including `**`) matched against changed file paths in the PR
+- `docs` — file paths or directory paths (all `.md` files under a directory) to inject
+- Docs are deduplicated across mappings
+
+### Architecture
+
+| Component | Description |
+|-----------|-------------|
+| `review/docmap.go` | YAML parsing, glob matching with `**` support, doc loading via VCS |
+| `cmd/review-bot/main.go` | Step 6c: parses config, intersects with changed files, calls LoadMatchingDocs |
+| `budget/budget.go` | New `DesignDocs` section — injected after Conventions in system prompt |
+| `action.yml` | `doc-map` and `doc-map-max-bytes` inputs, wired to `DOC_MAP_FILE`/`DOC_MAP_MAX_BYTES` |
+
+### Doc file loading
+
+- The `doc-map` YAML file is read from the local workspace (like `system-prompt-file`).
+- Doc files listed in the config are fetched via VCS API (same as `conventions-file`),
+  enabling them to be loaded from any branch without a local checkout.
+- `GetAllFilesInPath` is tried first; if it returns files, they are treated as a directory listing.
+  If it returns empty, `GetFileContent` is tried as a fallback (single file).
+
+### Glob matching
+
+`**` is implemented by splitting patterns and paths on `/`, then matching segment-by-segment.
+A `**` segment consumes zero or more path segments (not just one level like `*`).
+
+### Budget integration
+
+`DesignDocs` is added to `budget.Sections` between `Conventions` and `FileContext`.
+Trim order: Patterns → Conventions → DesignDocs → FileContext → Diff.
+Design docs appear in the system prompt under `## Design Documents`.
+
+### Context size guard
+
+Default: 100 KB. Configurable via `--doc-map-max-bytes` / `DOC_MAP_MAX_BYTES`.
+Truncation is noted inline with a `⚠️` message.
+
+## Error handling
+
+| Situation | Behavior |
+|-----------|----------|
+| `--doc-map` file not found | Fatal error (like `--system-prompt-file`) |
+| `--doc-map` file invalid YAML | Fatal error with descriptive message |
+| Unknown YAML keys | Log warning, continue |
+| Doc file not found in VCS | Log warning, skip |
+| Doc directory empty or no `.md` files | Log debug, skip |
+| Total size exceeds limit | Truncate with notice, log warning |
+| No changed paths match any mapping | No docs injected, review runs normally |
+| `paths` or `docs` list empty in a mapping | Skip that mapping |