feat(commands): quoted arguments and {{ARGUMENTS}} for slash commands

[nfertigsw]

Summary

Command templates already took positional {{name}} placeholders, but the invocation was split on whitespace with no notion of quoting, and nothing could capture a free-form instruction. Both gaps send the user back to editing the command every time they want to vary it:

/revue src/a.ts "gestion des erreurs"   →  {{angle}} got `"gestion`, the rest was dropped
/note rerun the flaky proxy test        →  no placeholder could hold this

Two additions, both in a command's Message template:

• Quoted values — wrap an argument in "…" or '…' and it fills one placeholder whole. Inside double quotes a backslash escapes the next character; single quotes are literal, as in POSIX shells. Touching fragments join, so src/"my file".ts is one token. An unterminated quote takes the rest of the line rather than erroring — someone mid-keystroke has one most of the time, and the inline hint reads the same buffer.
• {{ARGUMENTS}} — receives everything typed after the command id, verbatim. Excluded from positional numbering so it never steals a token another placeholder was waiting for, and case-sensitive: {{arguments}} stays an ordinary slot.

Both flow through the existing unfilled-placeholder path, so a command invoked bare still opens the params modal instead of shipping {{file}} to the model.

Full design rationale and edge cases: docs/DESIGN-SLASH-ARGUMENTS.md (added by this PR).

One implementation instead of two

Parsing lived in two near-duplicates: web/src/lib/parse-slash-command.ts for the chat composer and src/server/tasks/slash.ts for commands seeded into task sessions — each with its own extractTemplateParams and its own split(/\s+/). Both now delegate to src/shared/slash-args.ts, reachable from web through the existing @shared alias.

That was the point of the server-side copy existing in the first place: a command typed in chat and the same command seeded into a task should expand identically. They now do so by construction rather than by two implementations agreeing.

Two surfaces that follow from the parser

• The composer's param=? hint counted arguments with split(/\s+/), so a quoted value made it skip ahead. It now counts with the tokenizer, and templateParamHints orders {{ARGUMENTS}} last regardless of where it sits in the template, so the hint walks the slots in the order they have to be typed. Computed server-side in routes/commands.ts, so the composer and the task editor get identical hints from one place.
• The command editor gained a line describing the argument forms — including the {{name}} form, which nothing documented anywhere before.

Backward compatibility

With no {{ARGUMENTS}} in the template, positional resolution is exactly what it was. With no quotes in the invocation, tokenization is exactly what it was. Every pre-existing test passes with only the two new fields (args, rest) added to the parse result.

Workflows share the tokenizer, so quoted values now work for workflow parameters too. {{ARGUMENTS}} does not apply to them — they declare typed parameters rather than a template.

What to test

• A multi-word value. Give a command two placeholders, e.g. Relis {{file}} en te concentrant sur {{angle}}. Send /cmd src/a.ts "gestion des erreurs" and confirm the second placeholder gets all three words. Without the quotes it should still take just the first word, as before.
• Free-form. Write a command whose whole body is Add a note: {{ARGUMENTS}}, then send /cmd rerun the flaky proxy test. The full sentence should land in the prompt.
• Mixed. Relis {{file}}. Consignes : {{ARGUMENTS}} — the first token fills {{file}} and {{ARGUMENTS}} still carries the whole line, the same duplication $1 + $ARGUMENTS produces elsewhere.
• Nothing typed. Send the same commands bare (/cmd) and confirm the params modal opens and asks, rather than the raw {{file}} reaching the model.
• The inline hint. Start typing /cmd and watch the grey name=? hint. It should advance one slot per token — a quoted value counts once, not once per word — and offer ARGUMENTS=? only after the positional slots.
• Nothing regressed. Any existing command with plain positional arguments should behave exactly as before.
• Tasks. Seed a task whose prompt is a slash command with a quoted argument; the session should start with the expanded prompt, matching what chat produces.

Tests

45 added. src/shared/slash-args.test.ts covers the tokenizer (quotes, escapes, touching fragments, unterminated quotes, empty quoted argument), parsing, hint ordering, and expansion. The two callers keep their own suites: src/server/tasks/slash.test.ts gains end-to-end resolveSlashLaunch cases for quoted arguments, {{ARGUMENTS}}, the mixed form, the bare-invocation refusal, and quoted workflow parameters; ChatInput.slash.test.tsx gains composer-level cases for a quoted value reaching the template and for an unfilled placeholder surviving to the modal.

_{Local run on WSL: npm run typecheck ✅, eslint src/ web/src/ ✅ (0 issues), npm run duplicate ✅ (0 clones), prettier --check ✅, e2e ✅ (335 passed), unit 5000 passed with one failure — inspect-proxy.test.ts > handles unreachable target gracefully. That test proxies to http://127.0.0.1:1 and expects a refused connection; on this machine the connect hangs instead (verified independently: a raw TCP connect to port 1 times out rather than being refused), so the proxy never answers and the test hits its 15s cap. It fails identically on a clean checkout without this branch. CI on ubuntu-latest should be unaffected.}

AI-Enhanced Development

Tell what models helped shape this PR:

• AI Models: Claude Opus 5 (1M context), via Claude Code

Cache Impact

Does this PR affect anything cached — system prompts, tool definitions, skills, or other context?

• No. Command templates are not part of the system prompt, the tool definitions, or the skill metadata that feed computeDynamicContextHash — an expanded command becomes an ordinary user message. This PR changes only how that message's text is assembled before it is sent, so nothing cached is invalidated or reshaped.