fix: pre-landing review fixes (27 specialist findings, 3 critical)

Specialist army findings, all quote-verified before fixing:

Security: careful force-push guard now catches git's plus-refspec force
syntax (git push origin +main carried force with no flag — silently allowed
before) and refspec-form targets (HEAD:main); default-branch matching is
tokenized FIXED-STRING comparison on the full branch path (slashed defaults
like release/2.0 work; no ERE interpolation), glob-safe via noglob. HIGH rm
tier is tokenized too: trailing long options (--no-preserve-root) and /* are
root-class. Stored evidence fingerprints are 40-hex re-validated before
reaching git argv. normalizeForDetection sweeps ALL Unicode format chars
(\p{Cf}: soft hyphens, bidi marks, tag chars) instead of five enumerated
zero-widths. The wiring scanner gains flagless gh pr/issue view patterns. The
release-body banner tripwire diffs against the fetched original so a hostile
pre-existing banner string can't permanently DoS doc updates. Ship/land
evidence checks now pass --expect-cmd (a green `echo ok` recorded under the
label can never mint FRESH); package.json stays allow-listed with the
residual documented.

Performance: gstack-wtree seeds its temp index by COPYING the real index
(stat cache preserved — measured 40x faster than read-tree seeding, identical
hash) with read-tree fallback; evidence uses findLast and one gstack-slug
spawn; the stream pump honors backpressure via drain; careful's pattern block
short-circuits before slug resolution when no pattern file exists.

Testing: the gh-failure envelope test was VACUOUS (killing PATH killed the
bun shebang before the code under test ran) — replaced with a PATH gh shim
that exercises the real branch, plus shimmed happy paths (issue/pr-body/
unparseable JSON); evidence check --all + empty ledger + non-numeric
--max-age (now a usage error, was silent fail-open) covered; HIGH-tier
variants pinned; hook analytics respect GSTACK_HOME so tests stop writing the
operator's real skill-usage.jsonl.

Maintainability: dead exit ternary removed; flagValue deduped into
bin-context; sentinel defusal derived from the banner constants (no invisible
literals — \u escapes only); scratch-repo git fixture extracted to
test/helpers/scratch-repo.ts (one hermetic incantation, three consumers);
shared gstack_hook_log_fire in hook-extract.sh; the dashboard/land diff-scoped
row lists are aligned (codex-review) and drift-pinned.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bin/gstack-issue-guard

@@ -21,6 +21,7 @@
 
 import { spawnSync } from "child_process";
 import { wrapUntrustedTrackerContent } from "../lib/tracker-guard";
+import { flagValue } from "../lib/bin-context";
 
 function gh(args: string[]): { ok: boolean; out: string; err: string } {
   try {
@@ -36,11 +37,6 @@ function fail(msg: string): never {
   process.exit(1);
 }
 
-function flagValue(args: string[], name: string): string | undefined {
-  const i = args.indexOf(name);
-  return i >= 0 ? args[i + 1] : undefined;
-}
-
 const [, , mode, ...rest] = process.argv;
 
 if (mode === "--stdin") {