feat(#141): add validate-docmap subcommand

Adds 'review-bot validate-docmap' for CI hard-fail on docmap coverage gaps.

Usage:
  git diff --name-only origin/main HEAD | \
    review-bot validate-docmap --docmap .review-bot/doc-map.yml --repo-root .

Flags:
  --docmap     (required) path to doc-map YAML file
  --repo-root  (optional, default '.') root for resolving docs: paths

Two checks, both always run:
1. Coverage: every stdin file must match at least one paths: glob.
2. Stale docs: every docs: entry must exist on disk under --repo-root.

Exit codes: 0=clean, 1=failures found, 2=usage/parse error.

Tests cover: clean pass, uncovered file, stale doc, both failures,
empty stdin, blank-line stdin, and duplicate docs: deduplication.

cmd/review-bot/validatedocmap.go

@@ -0,0 +1,139 @@
+package main
+
+import (
+	"bufio"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"gitea.weiker.me/rodin/review-bot/review"
+)
+
+// runValidateDocmap implements the `review-bot validate-docmap` subcommand.
+//
+// It reads changed file paths from stdin (one per line, as produced by
+// `git diff --name-only`), parses a doc-map YAML file, and performs two checks:
+//
+//  1. Coverage check: every changed file must be matched by at least one
+//     paths: glob in the docmap. Fails if any file is uncovered.
+//
+//  2. Stale-docs check: every docs: entry in the docmap must exist on disk
+//     (relative to --repo-root). Fails if any path is missing.
+//
+// Both checks always run — all failures are reported before exiting.
+//
+// Exit codes:
+//
+//	0 — clean (all files covered, all docs exist)
+//	1 — one or more coverage or stale-doc failures
+//	2 — usage error, missing flag, or YAML parse error
+func runValidateDocmap(args []string) int {
+	fs := flag.NewFlagSet("validate-docmap", flag.ContinueOnError)
+	fs.SetOutput(errWriter)
+
+	docmapFlag := fs.String("docmap", "", "Path to doc-map YAML file (required)")
+	repoRootFlag := fs.String("repo-root", ".", "Repo root for resolving docs: paths (default: cwd)")
+
+	if err := fs.Parse(args); err != nil {
+		// flag.ContinueOnError already wrote the error to errWriter.
+		return 2
+	}
+
+	if *docmapFlag == "" {
+		fmt.Fprintln(errWriter, "Error: --docmap is required")
+		fmt.Fprintln(errWriter, "")
+		fmt.Fprintln(errWriter, "usage: review-bot validate-docmap --docmap <path> [--repo-root <dir>]")
+		fmt.Fprintln(errWriter, "  Changed files are read from stdin, one per line.")
+		fmt.Fprintln(errWriter, "  Example: git diff --name-only origin/main HEAD | review-bot validate-docmap --docmap .review-bot/doc-map.yml")
+		return 2
+	}
+
+	// Parse docmap YAML.
+	cfg, err := review.ParseDocMapConfig(*docmapFlag)
+	if err != nil {
+		fmt.Fprintf(errWriter, "Error: failed to parse docmap %q: %v\n", *docmapFlag, err)
+		return 2
+	}
+
+	// Read changed files from stdin.
+	changedFiles, err := readLines(os.Stdin)
+	if err != nil {
+		fmt.Fprintf(errWriter, "Error: failed to read stdin: %v\n", err)
+		return 2
+	}
+
+	failed := false
+
+	// --- Check 1: Coverage ---
+	var uncovered []string
+	for _, f := range changedFiles {
+		if !review.FileCoveredByDocMap(cfg, f) {
+			uncovered = append(uncovered, f)
+		}
+	}
+	if len(uncovered) > 0 {
+		failed = true
+		fmt.Fprintln(errWriter, "ERROR: changed files with no docmap coverage:")
+		for _, f := range uncovered {
+			fmt.Fprintf(errWriter, "  %s\n", f)
+		}
+	}
+
+	// --- Check 2: Stale docs ---
+	repoRoot := filepath.Clean(*repoRootFlag)
+	staleDocs := checkStaleDocs(cfg, repoRoot)
+	if len(staleDocs) > 0 {
+		failed = true
+		fmt.Fprintln(errWriter, "ERROR: stale docmap docs: entries (paths do not exist):")
+		for _, d := range staleDocs {
+			fmt.Fprintf(errWriter, "  %s\n", d)
+		}
+	}
+
+	if failed {
+		return 1
+	}
+
+	fmt.Fprintln(outWriter, "OK: docmap is valid")
+	return 0
+}
+
+// checkStaleDocs returns deduplicated docs: entries that do not exist under repoRoot.
+func checkStaleDocs(cfg *review.DocMapConfig, repoRoot string) []string {
+	seen := make(map[string]struct{})
+	var stale []string
+
+	for _, mapping := range cfg.Mappings {
+		for _, docPath := range mapping.Docs {
+			if docPath == "" {
+				continue
+			}
+			if _, ok := seen[docPath]; ok {
+				continue
+			}
+			seen[docPath] = struct{}{}
+
+			fullPath := filepath.Join(repoRoot, filepath.FromSlash(docPath))
+			if _, err := os.Stat(fullPath); err != nil {
+				stale = append(stale, docPath)
+			}
+		}
+	}
+	return stale
+}
+
+// readLines reads all non-empty trimmed lines from r.
+func readLines(r io.Reader) ([]string, error) {
+	scanner := bufio.NewScanner(r)
+	var lines []string
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if line != "" {
+			lines = append(lines, line)
+		}
+	}
+	return lines, scanner.Err()
+}