fix(#2978): add structural validation to roadmap validate (#3092)

* test(#2978): roadmap validate must perform structural validation

Failing-first: roadmap validate returns {"warnings":[]} (exit 0) for every
input — empty file, garbage, missing file, truncated frontmatter — because it
performs no structural validation and its one opt-in check (W021 milestone-
prefix) is off by default. Six cases: empty, garbage, missing, truncated
frontmatter, well-formed (no false positive), BOM-prefixed (not corruption).

* fix(#2978): add structural validation to roadmap validate

roadmap validate returned {"warnings":[]} (exit 0) for every input — empty
file, garbage, missing file, truncated frontmatter — because it performed no
structural validation and its one opt-in check (W021 milestone-prefix) is
off by default. A verb named validate that cannot produce a negative result
provides false assurance.

Add four structural checks, each producing a coded warning {code, message}:
- V001: file missing/unreadable (was silent success)
- V002: empty/whitespace-only
- V003: malformed frontmatter (unterminated --- fence; BOM-tolerant per #3057)
- V004: no recognizable phase entries (no ### Phase N: heading)
Keep the existing W021 milestone-prefix check as-is. Exit non-zero via
ExitError(1) when warnings are non-empty, per the documented contract
('exits non-zero on any error or warning'). Well-formed roadmaps (incl.
BOM-prefixed, CRLF) still validate cleanly with warnings: [] and exit 0.

* test(#2978): update W021 tests for non-zero exit on warnings

Two existing W021 tests asserted roadmap validate exits 0 even with warnings
('roadmap validate should exit 0 even with warnings') — that was the bug.
#2978 made validate exit non-zero on any warning (per its documented
contract). Updated both mismatch-case tests to expect success===false and
parse the JSON output from the failure path (stdout is written before the
ExitError throw).

* chore(#2978): add changeset fragment

* chore(#2978): backfill changeset PR number 3092

---------

Co-authored-by: sim <sim@local>

tests/milestone-prefixed-convention.test.cjs

@@ -89,7 +89,8 @@ describe('W021 — milestone-prefixed phase ID convention', () => {
     ]);
 
     const result = runGsdTools(['roadmap', 'validate'], tmpDir);
-    assert.ok(result.success, `roadmap validate should exit 0 even with warnings: ${result.error}`);
+    // #2978: validate now exits non-zero on any warning (per its documented contract).
+    assert.strictEqual(result.success, false, `roadmap validate must exit non-zero on W021 warnings: ${result.error}`);
 
     const out = JSON.parse(result.output);
     assert.ok(Array.isArray(out.warnings), 'output.warnings should be an array');
@@ -219,7 +220,8 @@ describe('W021 — milestone-prefixed phase ID convention', () => {
     ]);
 
     const result = runGsdTools(['roadmap', 'validate'], tmpDir);
-    assert.ok(result.success, `roadmap validate failed: ${result.error}`);
+    // #2978: validate now exits non-zero on any warning (per its documented contract).
+    assert.strictEqual(result.success, false, `roadmap validate must exit non-zero on W021 warnings: ${result.error}`);
 
     const out = JSON.parse(result.output);
     const w021 = (out.warnings || []).filter(w => w.code === 'W021');