CL-6499: Fix /<agent> slash-command grammar regression, friendlier unknown-command message

[TheGreatAxios]

Summary

The owner asked for typing /jimmy in workbench chat to invoke that agent directly, the same way @Jimmy does. Turns out this mechanism already exists and is already wired up end-to-end — no new command system needed:

• packages/commands already has a generic /name args and @name args grammar, a command registry, and createWorkflowCommandPlugin, which derives one command per tenant per invitable agent definition (never a hardcoded list — CL-6451/CL-6471 already built this).
• packages/chat/src/routes.ts's dispatchWorkbenchCommand already intercepts a leading / on an incoming message and dispatches through that registry.
• startWorkflowCommand / forcedRecipientAddress in packages/chat/src/workbench-service.ts already routes a command to a resident agent's existing run, or invites+launches it.
• The chat-ui composer already forwards any typed text starting with / straight through as a message when it doesn't match one of the composer's own fixed UI commands (/invite, /summarize, /routine, /agents, /help) — so /jimmy hi was already reaching the backend command intercept with no client change needed.

So /jimmy throw me a gif for shipping code already worked today, as long as an agent named jimmy is invitable in the tenant. This PR fixes two real gaps found while verifying that:

1. Regression fix: packages/commands/src/grammar.ts's parseWithPrefix only split on the first space — it never validated the command-name shape. A plain message that merely starts with / (a filesystem path like /usr/local/bin, a URL) was parsed as a command name and, on no match, answered with a raw "Unknown command: /usr/local/bin" message that replaced the user's own text on the timeline. Command names are now constrained to [\w-]+ — the same shape the composer's own activeSlashQuery already commits to — so a path-shaped message now posts normally.
2. Friendlier unknown-command message: dispatch.ts's unknown-command result used to be a bare "Unknown command: /nope". It now names what commands ARE available ("Unknown command: /nope. Available: /jimmy, /scout.", or a "no agent commands are available yet" message when the tenant has none) — never a silent no-op, never a raw error with no next step.

What I checked before building anything

• packages/chat-ui/src/slash-commands.ts — the composer's own fixed UI-command catalog (invite/summarize/routine/agents/help). Confirmed it does NOT intercept unmatched /word text; that falls through to an ordinary send().
• packages/chat/src/mentions.ts + workbench-service.ts's forcedRecipientAddress — the hook the task suggested. Already exactly what startWorkflowCommand uses.
• packages/agent-directory — confirmed the command registry derives from listInvitableDefinitions per tenant, not a hardcoded agent list.
• packages/commands — the actual command grammar/registry/dispatch package, already fully wired into the hub (apps/hub/src/index.ts registers createWorkflowCommandPlugin).

Discoverability/autocomplete: deferred, per the task's own escape hatch. The composer's slash popover only lists its fixed 5 UI commands today; wiring it to also list per-tenant agent commands (fetching GET /commands, merging, handling arbitrary execution results client-side) is a larger change than this timebox allows. Working invocation already existed and is now regression-fixed; discovery is left as a follow-up.

Tests (red then green, tests-first commit)

• packages/commands/test/grammar.test.ts — path-shaped slash text (/usr/local/bin) parses to undefined; hyphenated names still parse.
• packages/commands/test/dispatch.test.ts — unknown command names available commands; path-shaped text never dispatches.
• packages/commands/test/workflow-commands.test.ts — /jimmy <text> routes to the definition named jimmy with the remainder as args (proves the agent-directory-derived, non-hardcoded path).
• packages/chat/test/commands.test.ts — full route-level test: /jimmy throw me a gif for shipping code → workflow-started for jimmy, mail delivered with that text; a path-shaped message posts normally and is never swallowed; unknown-command message updated.

Verification (partial — see note)

Per an urgent note mid-task: the owner's machine OOM'd from concurrent lanes running full-monorepo bun run check. I did not run repo-root bun run check or repo-root lint.

Ran only, package-scoped, one at a time:

• packages/commands: bun test → 30 pass, 0 fail
• packages/chat: bun test → 640 pass, 24 skip, 0 fail (full package suite, not just the touched file)

Not run: repo-root typecheck, repo-root lint, full-workspace bun run check. CI should cover these.