chore(tui): remove 12 dead hint/todo/session/checkpoint i18n keys (r66, pool final batch)

[topcheer]

r66 dead-code scan — i18n pool final batch (four families)

Base: d631d9058 (origin/main after #4324; r63 #4320 already merged).

Removed — 12 keys × 10 language catalogs (120 case/return pairs, −240 lines)

• hint (5): hint.add_context, hint.ctrlv_image, hint.enter_send, hint.image_attached, hint.scroll
• todo (4): todo.none, todo.parse_failed, todo.read_failed, todo.title
• session (2): session.list_failed, session.none (r63's removed list-trio not confused — these two verified independently)
• checkpoint (1): checkpoint.none

Verification (five-channel)

1. Full-repo grep (prod code, all packages incl. desktop/cmd): zero references — keys appear only in the 10 catalog definitions
2. Test channel: zero references in *_test.go (only hint.help/mode/ctrlc_exit/ctrlr_sidebar/shift_tab_mode referenced there — all live)
3. Dynamic-prefix channel: no "hint." + / "todo." + / "session." + / "checkpoint." + construction anywhere
4. Non-Go channel: zero refs outside .go files
5. case/return pairing guard: script asserted each of 120 targets is a direct case→return pair (first pass caught indentation mismatch, 0 warnings on final pass)

Post-clean validation

• go build -tags goolm ./... ✅
• go vet -tags goolm ./internal/tui/... ✅
• go test -tags goolm ./internal/tui/ -run TestI18N ✅ (completeness parity kept — keys removed uniformly from all 10 catalogs)

Note on 13→12

Task brief said 13 keys (hint×6); on refreshed base only 12 four-family keys are dead (hint×5). 6th hint candidate hint.mention is live (view_chat.go:236, reference predates v1.2.8) — not touched.

Does not overlap in-flight #4317 (r61 mode.), #4321 (r64 panel.qq.), #4325 (r65 label/memory/tool/hooks/agents) — disjoint key sets.

[topcheer]

[ggcxf 走读] GO：12 死键（hint/todo/session/checkpoint 族）×10 语言目录对称移除，+0/-240 纯删除，抽样每键恰 10 处一致，无逻辑面（#4317/#4321/#4325 同族 final batch）。门面：CI Test=pending；approvals=0（ggaidev=read 通道断损同根因）。CI 绿+审批在案后进合并判定。

[topcheer]

[ggcxf 更新] CI 已全绿（9/9，Test=pass）。走读 GO 在案，仅剩审批面阻塞（approvals=0，ggaidev=read 通道未修复），留治理处置。