feat(web-ui): precise slash-command matcher

[jarvis24young]

What this PR does

Replaces the nine loose text.startsWith('/btw') checks scattered across
ChatInput.tsx with a single isSlashCommand(text, command) helper that
enforces a proper word boundary. This fixes two real bugs in one place.

Bug 1 — word-boundary miss (the headline fix)

Before this PR, the chat composer used text.toLowerCase().startsWith('/btw')
to detect the /btw command. That predicate has no word boundary, so
every command that happened to start with the same prefix was
accidentally dispatched as that command.

User types	Before this PR	After this PR
/btw 你好	✅ /btw (correct)	✅ /btw (correct)
/btwextra 你好	❌ /btw 你好 (sent to btw side-session)	✅ generic message
/goals 我的目标	❌ /goal 我的目标 (entered goal mode!)	✅ generic message
/compactor	❌ /compact (showed usage error)	✅ generic message
/usage2	❌ /usage (showed usage error)	✅ generic message

Bug 2 — Claude Code 2.1.147: trailing tab / newline

A slash command followed by a stray tab or newline used to be rejected
by the strict regexes in the Send path. The new helper uses the
lookahead (?=\s|$), which treats tab and newline as valid trailing
whitespace.

User types	Before this PR	After this PR
/goal 修 bug\t	❌ unknown command	✅ /goal 修 bug
/btw\n	❌ unknown command	✅ /btw
/btw hello	✅ /btw hello	✅ /btw hello (unchanged)

How it works

A new module at src/flow_chat/utils/slashCommand.ts exports three
helpers:

matchesSlashCommand(text)        // → '/btw' | '/goal' | null
isSlashCommand(text, command)    // → boolean, with word boundary
stripSlashCommand(text, command) // → argument portion, trims trailing WS

The internal regex is ^(\/[a-zA-Z][\w:-]*)(?=\s|$)/ — the
(?=\s|$) lookahead is what enforces a proper boundary at the first
whitespace or end-of-string.

All nine existing call sites in ChatInput.tsx are routed through
isSlashCommand. The scope covers every detection path:

• handleInputChange (3 sites) — picker activation gating
• handleSendOrCancel (5 sites) — Send-button dispatch
• Enter-key handler (1 site) — direct keyboard submit

The call at ChatInput.tsx:2005
(message.toLowerCase().startsWith('/goal') && !isGoalSlashCommand(message))
is deliberately kept as a loose check — it is the fallback that
shows "Use /goal with optional focus text…" when the user typed
something like /goals foo that almost matches but is malformed.

Tests

20 unit tests in src/flow_chat/utils/slashCommand.test.ts:

Test Files  1 passed (1)
     Tests  20 passed (20)

Coverage includes:

• All four happy-path cases (/btw, /goal, /usage, /DeepReview)
• All 2.1.147 trailing-whitespace cases (tab, newline, CRLF)
• All word-boundary false-positive cases (/btwextra, /goals, /compactor, /usage2)
• MCP prompt commands with : and - in the name
• Defensive non-string inputs
• Regex metacharacter handling

Code review

Reviewed by an independent codex review pass. The two highest-severity
findings were:

1. Initial PR only fixed 2 of 9 call sites (HIGH). Resolved by
   expanding the PR to cover all 9 in this same commit.
2. Test coverage gap on regex metacharacter escape (LOW). Closed
   with the metacharacter-isolation test.

All other findings were LOW (JSDoc nit on \t/\n reading as
literals; command: string could be a tighter /${string} template
literal type) and are documented but not blocking.

Files changed

src/web-ui/src/flow_chat/utils/slashCommand.ts          (new,  94 lines)
src/web-ui/src/flow_chat/utils/slashCommand.test.ts    (new, 153 lines)
src/web-ui/src/flow_chat/components/ChatInput.tsx      (modified, +15/-10)

Test plan for review

• cd src/web-ui && npx vitest run slashCommand → 20/20 pass
• cd src/web-ui && npx tsc --noEmit → 0 errors
• Manually open BitFun, type /btwextra hello, press Enter → message
  is dispatched as a generic user message (NOT to a btw side-session)
• Type /goals my goal, press Enter → message dispatched as a generic
  user message (NOT goal mode)
• Type /btw\t, press Enter → opens a btw side-session as expected
• Type /goal 修 bug, press Tab (in a non-shift-target context), press
  Enter → enters goal mode as expected

[wsp1911]

Thanks for tightening the slash-command matcher. The helper and focused tests look good, and the /btwextra / /usage2 cases are exactly the kind of regression coverage we want here.

One merge-sensitive issue: this PR is currently behind main, and the conflicted area in ChatInput.tsx overlaps with ACP slash-command handling. main added localSlashCommandsEnabled = !isAcpInputSession gates so ACP sessions can pass their own slash commands through instead of being captured by local /btw, /compact, /usage, etc. When resolving the conflict, please keep those gates around the new matcher, e.g. localSlashCommandsEnabled && isSlashCommand(...).

Small consistency issue: there is still one loose /goal prefix check in the send-button path:

message.toLowerCase().startsWith('/goal') && !isGoalSlashCommand(message)

The normal keyboard flow for /goals shows “no matching command” in the picker, but if the picker is still open and the user clicks the send button, this branch fires and shows the /goal usage warning. That preserves the old prefix-conflation behavior for this click path. Could we align this branch with the new boundary matcher as well?