docs(specs): add TUI inline mention picker spec, relocate busy status to input separator

[bug-ops]

Summary

Specification-only PR for the TUI UX improvement epic: inline @ mention picker and status-line stabilization. No code changes.

New spec

• specs/084-tui-mention-picker/spec.md [approved] — inline non-modal @ popup for files/skills/agents replacing the modal file picker:
  • word-start trigger: @ opens the popup only at position 0 or after whitespace; mid-word @ (e-mail addresses, pkg@version) stays a literal character
  • typing is never captured away from the input buffer; the popup reflects the @query span between the sigil and the cursor
  • Space closes the popup keeping the mention text as plain prose; Tab/Enter accept the selection plus a trailing space; Esc closes only the popup and retains Insert mode; Backspace past the @ dismisses
  • category tabs All/Files/Skills/Agents switched with Left/Right while open; nucleo fuzzy matching with match highlighting and an N/M counter
  • per-category accept semantics: file = bare repo-relative path, skill = plain name (no forced activation), agent = @name sigil retained so the existing dispatch_agent_command routing applies
  • race-free file-index build (placeholder row instead of the current input-loss window); skill/agent catalog delivery via a dedicated startup/hot-reload event — no per-tick MetricsSnapshot bloat, no new zeph-skills dependency in zeph-tui

Updated specs

• specs/011-tui/spec.md — the canonical location of the transient busy spinner+verb moves from the bottom status bar to the input separator row; the bottom bar holds stable session parameters, streaming tok/s+TTFT, and transient critical hints; Spinner Rule itself unchanged
• specs/030-tui-slash-autocomplete/spec.md — NFR-004 rewritten: non-conflict between the two inline surfaces via distinct trigger rules
• specs/tui-reducer/spec.md — FilePicker* action set marked superseded by MentionPicker* variants (removed at implementation time; INV-R1/INV-R2 unchanged)
• specs/UX/mention-routing.md — addendum: the TUI-side blocker for @agent mentions is resolved by spec 084; routing invariants remain binding; A2A-remote mentions stay deferred
• specs/README.md, specs/MOC-specs.md — spec 084 registered

Coordination

• Interrupt-hint key/text on the separator row is owned by require double Ctrl+C to exit TUI and move agent-cancel from Esc to Ctrl+C #6646 (esc to interrupt today, ctrl+c once it lands); this epic does not touch Ctrl+C/Esc semantics.

Implementation is tracked separately:

• modal @ file picker blocks prompt typing and swallows literal @ characters #6647 (P1, inline picker)
• @ picker cannot search skills or agents; typed @agent mentions never reach the backend router from the TUI #6648 (P2, skill/agent categories)
• variable-width busy status shifts bottom status bar segments; spinner duplicated between status bar and input separator #6649 (P2, status relocation)
• slash autocomplete and @ file picker use different fuzzy engines #6650 (P3, fuzzy-engine unification)
• @ picker with empty query lists alphabetical index head instead of recently modified files #6651 (P3, empty-query ordering)

Verification

• Spec-only change (specs/*.md); no compiled target, rustdoc input, or CI feature set is affected — matching prior spec-only PRs docs(specs): correct stale references, broken links, and formatting #6629/docs(specs): add Telegram rich-text formatting sub-spec for Bot API 10.1/10.2 #6542
• gitleaks protect --staged: no leaks
• Cross-reference check: no stale [[...]] links; spec numbering verified against existing 081–083 directories