bug(cli): unrecognized arguments create sessions instead of erroring

[OneStepAt4time]

Bug Report — CLI Dogfooding

Command: ag foobar (or any unrecognized argument)
Expected: Error: "Unknown command: foobar. Run ag --help for usage."
Actual: Creates a session with prompt "foobar".

Reproduction

$ ag foobar
  ✅ Session created: cc-foobar
     ID: 3e3fa9b3-ad19-4a71-a692-54b84139b050
  ✅ Brief delivered (attempt 1)

Problem

The CLI treats any unrecognized argument as a session brief via the shorthand ag "brief" syntax. This means typos like ag statis, ag list, ag stop, ag status all create unwanted sessions instead of showing an error.

This contradicts the intent of PR #3035 which added --reject-unknown-commands to the CLI.

Suggested Fix

Only treat arguments as briefs if they look intentional (e.g., quoted strings or sentences). Single-word arguments that match common command-like patterns should be rejected with a helpful error.

Severity

P2 — creates zombie sessions from typos, confusing for new users.

[aegis-gh-agent]

Documented as known issue in PR #3390 (merged on 2026-05-14). Squash merged as 0b49c1f. Troubleshooting entry added explaining arg shorthand behavior. Code fix still needed.

[OneStepAt4time]

Already fixed in commit 3f51c7e (PR #3079) which is on develop. Single-word unrecognized args now print Unknown command: "foobar". Run ag --help for usage. and exit with code 1.

Multi-word bare args (containing spaces) are still treated as prompts for backward compat — this is intentional behavior for the ag "describe your task" shorthand.