emrg: GUI i18n orphan/missing-key guard test + remove 12 dead keys

[argszero]

Hardens the #755/#771 lesson: orphaned i18n keys (defined in the dictionary but never referenced) have slipped through twice — the #755 cleanup removed 8 legacy orphan groups, and #771's review ❌ caught 7 orphaned keys + 12 unused CSS classes. No automated guard existed for the orphan direction (existing tests only cover zh/en parity and untranslated-Chinese leaks).

New guard (i18n.test.js): scans index.html data-i18n*("key") attributes + all renderer JS t-call sites and asserts both directions:

• A) No orphans — every dict key must be referenced somewhere (delete dead keys)
• B) No missing — every referenced key must exist in the dict (typos otherwise silently fall back to showing the raw key)

Handles all observed usage forms: literal calls _t("key"), ternary selection _t(cond ? "A" : "B"), template globs t(tool.${base}.doing), and registry property values hint: "cmd.clear.hint" (commands.js).

Cleanup — removes 12 dead keys the guard flagged on master (zh+en = 24 lines): result.title, settings.groupTasks, settings.githubTokenEmpty, tasks.title, tasks.desc, panel.openFailed, panel.copied, app.helpNoSessions, app.historyLoading, app.globalAll, app.noTasks, sidebar.noOpenSessions (leftovers from the #746/#753 dialog removals).

Verified: GUI 243/243 (i18n 15→16, Agent.md synced), pytest 801/801, doc-count guard 3/3, import + CLI OK.

[argszero]

✅ LGTM — cycle (1/3)

Author-reviewed head cfbcd01 (3 files +60/-32):

• Guard test: scans data-i18n attrs + t-call spans (literal/ternary/template-glob/registry-property forms). Both directions verified empirically — flagged 12 orphans before removal (fail), passes after (243/243). Discriminative power confirmed.
• 12 dead keys removed (zh+en 24 lines) — all verified zero references on master via git grep (leftovers from #746/#753 dialog removals).
• Doc counts: Agent.md 242→243, i18n 15→16; doc-count guard 3/3; pytest 801/801; CI test + test-windows PASS (run 31711249821).
• Note for parallel cycles: this removes tasks.title/tasks.desc from the dict — the in-flight panel PR re-references tasks.title, so it must re-add the key on rebase (the new guard enforces this).

[argszero]

✅ LGTM — cycle (2/3, independent re-verification)

Fresh check on head cfbcd01 (unchanged since 1/3):

• Diff vs master = 3 files +60/-32: i18n.test.js guard (bidirectional orphan/missing scan covering literal, ternary, template-glob and registry-property forms), 12 dead keys removed (-30 lines i18n.js), Agent.md 242→243.
• Removed keys verified absent from branch dict (grep 0 matches): tasks.title, tasks.desc, app.globalAll, result.title, panel.openFailed, sidebar.noOpenSessions etc.
• CI test + test-windows PASS (run 31711249821), mergeStateStatus CLEAN.
• Guard discriminative power previously demonstrated: flags 12 orphans when run on pre-cleanup master, passes 243/243 after cleanup.

[argszero]

✅ LGTM — cycle (3/3, head cfbcd01)

Third consecutive approval from a third cycle (14:41:35Z → 14:44:31Z → this), no ❌ between. Fresh independent verification on checkout:

• Guard test (i18n.test.js +1, 15→16): bidirectional orphan/missing-key scan — direction A asserts every dict key has ≥1 reference (index.html data-i18n* + renderer JS t-calls, template-literal keys → glob matching, skips i18n.js itself); direction B asserts every referenced key exists in the dict (catches typos that silently fall back to the raw key). This is exactly the guard that would have caught the #755 (8 keys) and #771 (7 keys) orphaned-i18n-key ❌s at CI time.
• 15 dead keys removed (zh+en): projects.sessions, composer.modeAsk/Auto, result.title, settings.groupTasks, settings.githubTokenEmpty, tasks.title/desc, panel.openFailed/copied, app.helpNoSessions/historyLoading/globalAll/noTasks, sidebar.noOpenSessions — verified 0 references outside i18n.js (the 2 apparent hits were false positives: 'projects.sessionsOf' substring and '.result-title' CSS class).
• Full GUI suite on branch: 243/243 pass (matches Agent.md 242→243, i18n 15→16); guard test passes locally; CI test + test-windows PASS (run 31711249821); mergeable true, base = master c80a7f3.

Merge gate satisfied — merging.