feat(zeph-tui): replace modal @ file picker with inline mention popup

[bug-ops]

Summary

• Replaces the modal @ file picker with a non-modal inline popup: typing @ at word-start inserts the character into the input buffer and opens a suggestion popup that reflects the buffer, instead of capturing every subsequent keystroke into a separate filter field (fixes the input-loss race and the inability to type a literal @).
• Adds category tabs to the popup: All | Files | Skills | Agents. Files reuses the existing background-built file index; Skills are delivered via a new Channel::send_skill_catalog event (emitted at agent startup and on skill hot-reload); Agents read directly from existing runtime metrics (MetricsSnapshot::agent_definitions), requiring no new plumbing.
• Accepting a selection is token-bounded: it replaces the whole @query word rather than just the text up to the cursor, so accepting with the cursor positioned inside an existing mention can no longer corrupt the buffer.
• reduce is now a thin wrapper around reduce_inner plus a fail-closed resync step that closes or refilters an open picker after any action that could plausibly touch the input buffer or session state — including paste and session switching, neither of which the old modal design could observe.
• specs/084-tui-mention-picker/spec.md amended in several passes to match the shipped design (this went through three review rounds specifically to close remaining spec/code drift).

Closes #6647
Closes #6648

Test plan

• cargo +nightly fmt --check
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins (15088 passed, 0 failed)
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• gitleaks protect --staged --no-banner --redact
• New regression tests cover the paste-buffer-mutation and session-switch cases that motivated the fail-closed resync design, and a dedicated AppChannel::Tui forwarding test for the new skill-catalog event (verified by revert-and-rerun to actually fail without the fix)
• Live-TUI verification (Skills tab populated from a real agent session, popup anchoring/highlighting in both themes) — see .local/testing/playbooks/tui-mention-picker.md Scenario F, not yet run