feat(#137): add doc-map input for path-scoped doc injection

- New --doc-map flag (DOC_MAP_FILE env var): path to YAML config mapping
  source path globs to governing design docs
- New --doc-map-max-bytes flag (DOC_MAP_MAX_BYTES env var): cap on total
  injected doc content, default 100KB
- review/docmap.go: DocMapConfig parsing, glob matching with ** support,
  doc loading via VCS with directory expansion and size guard
- budget.Sections: new DesignDocs field, trimmed after conventions
- budget.buildResult: injects DesignDocs under ## Design Documents heading
- action.yml: doc-map and doc-map-max-bytes inputs wired to env vars
- CHANGELOG.md: created with unreleased entry
- Tests: ParseDocMapConfig, MatchDocs, globMatch, LoadMatchingDocs

budget/budget.go

@@ -63,7 +63,8 @@ type Sections struct {
 	SystemBase  string // Core instructions (never trimmed)
 	Patterns    string // Language patterns (trimmed first)
 	Conventions string // Repo conventions (trimmed second)
-	FileContext string // Full file content (trimmed third)
+	DesignDocs  string // Path-scoped design documents (trimmed third)
+	FileContext string // Full file content (trimmed fourth)
 	Diff        string // The actual diff (trimmed last, only truncated)
 	UserMeta    string // PR title, description, CI status (truncated only if base exceeds budget)
 }
@@ -103,6 +104,7 @@ func Fit(model string, sections Sections) Result {
 	entries := []entry{
 		{"patterns", &sections.Patterns},
 		{"conventions", &sections.Conventions},
+		{"design docs", &sections.DesignDocs},
 		{"file context", &sections.FileContext},
 	}
 
@@ -185,6 +187,10 @@ func buildResult(s Sections, trimmed []string, estTokens int) Result {
 		sys.WriteString("\n\n## Repository Conventions\n\nThe repository has the following coding conventions that must be respected:\n\n")
 		sys.WriteString(s.Conventions)
 	}
+	if s.DesignDocs != "" {
+		sys.WriteString("\n\n## Design Documents\n\nThe following design documents govern the changed code. Review the diff for adherence:\n\n")
+		sys.WriteString(s.DesignDocs)
+	}
 
 	var usr strings.Builder
 	usr.WriteString(s.UserMeta)