feat(tui): @-mention autocomplete for file/folder context input

[quangdang46]

Summary

Implements @<path> autocomplete in the TUI input with Claude Code parity. Type @ anywhere in chat → dropdown shows file/folder suggestions ranked by frecency; pick a folder to drill in, pick a file to inline its content on submit.

What works

• @ triggers picker dropdown anywhere in input (cursor-based detection, not just end-of-input)
• File suggestions insert @path and inline full content on submit (cap: 200 KB)
• Folder suggestions insert @path/ and KEEP the dropdown open for drill-in; on submit, expand to a 1-level directory listing (cap: 1000 entries, dirs sorted before files)
• Quoted paths @"path with spaces" for paths containing spaces
• Line ranges @file#L10 and @file#L10-20
• Multiple mentions per message all expanded; email-like user@example.com correctly skipped
• Frecency-ranked results — repeatedly-mentioned paths float to the top, persisted to ~/.jcode/cache/ffs/<repo-hash>/

How it works

Three layers, one per commit:

1. AtPicker wraps ffs-search 0.1.3. Background scan + filesystem watcher rooted at session.working_dir, gitignore-aware via ffs's ignore integration, fuzzy match via neo_frizbee, frecency persisted via LMDB. Lazy AtPickerSlot state machine (Pending/Initialized/Failed) so we don't spawn threads in test/replay modes.

2. Cursor-based token detection in state_ui_input_helpers.rs. New extract_at_token_at_cursor(input, cursor) → Option<AtTokenMatch> returns byte offsets so callers can do exact in-place replacement. Handles quoted paths, unicode (CJK, accents), multiple @s in one input. The command_suggestions() entry point checks for an active @-token first; if found, suggestions come from AtPicker::search() and the slash/dollar branches don't run. The autocomplete() (Tab) handler does the in-place token replacement: file → trailing space (close), folder → trailing slash (drill-in), space-containing paths → quoted form.

3. Submission-time expansion in input.rs. expand_at_path_references() rewritten as a byte-index state machine. Folders inline a sorted listing (matches Claude Code's behavior — no recursive content inlining, just direct children with truncation note). Line ranges parsed off the end of the token so paths containing # work. Invalid ranges leave the token literal so the user can fix the typo.

Spec source

UX choices match claude-code-best/claude-code source (verified, not docs):

• MAX_DIR_ENTRIES = 1000 — src/utils/attachments.ts:1973
• applyDirectorySuggestion: file → ' ', dir → '/' — src/hooks/useTypeahead.tsx:277-279
• 1-level readdir, no recursive content inlining — src/utils/attachments.ts:1968-1995
• MAX_SUGGESTIONS = 15 — src/hooks/fileSuggestions.ts

Tests

41 new tests across 3 modules:

Module	Tests	What's covered
at_picker::tests	8	state machine, search, dirs-only mode, idempotent warm_up, multi-picker no-deadlock, stable cache keys
state_ui_input_helpers::dollar_token_tests (cursor section)	21	start/middle/end positions, multiple consecutive tokens, email rejection, quoted forms (closed/unclosed/cursor-mid-quote), unicode paths, slash separators, whitespace boundaries
input::at_path_tests	12	quoted paths, line ranges (single/multi/clamped/invalid), folder listing sort, folder cap + truncation note, multiple mentions, email bystander, folder w/o trailing slash

Plus 26 pre-existing tests in those modules unchanged and passing.

Verification

• cargo check --lib ✅
• cargo check --tests ✅ (compiles every test in the workspace, no regressions elsewhere)
• Targeted test runs for the 3 modules ✅ (66/66 passing)

Out of scope (separate PRs)

• MCP resource mentions @server:resource/path
• Agent mentions @agent-name
• Drag-drop file → terminal
• Image attachment via @image.png
• ffs-search version bump strategy (currently pinned =0.1.3)