fix(rig-acp): rig acp refuses an option after acp instead of ignoring it

Description

From the 2026-11-07 architecture audit (K3-A, f-1). Invocation::parse puts every word after a command into its arguments. rig-acp reads only the options before acp (Invoked::flag(TOOLS)) and never reads the arguments. So rig acp --tools=read started an ACP server whose sessions ran with every tool, not only the ones the user asked for, and nothing said so.

The router now refuses at startup when any word follows acp. It says why on stderr and exits with code 2, through the usual refuse_run path. An editor that launches rig acp --tools=read sees:

rig: rig acp takes nothing after `acp`, so `--tools=read` would be ignored; put options before `acp`, as in `rig --tools=read acp`.

Why in rig-acp, not the shared parser. rig acp takes no words, so refusing any word there leaves every other command's parsing as it is. Changing Invocation::parse to read options after a command would change all commands, and their words are the command's to read.

What users see change

• rig acp --tools=read (or any word after acp) no longer starts an ACP server. It exits with code 2 and prints on stderr that nothing is taken after acp and options go before it, as in rig --tools=read acp.
• rig --tools=read acp (options before acp) works as before.

Changelog

• (rig-acp) rig acp refuses any word after acp, such as rig acp --tools=read, saying to put options before acp, instead of ignoring it.

Migration

None. A command line that put options after acp was already ignoring them. It now has to put them before acp.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update

Testing

• cargo nextest run --locked --profile local -p rig-acp on box2: 37 of 37 passed
• cargo clippy --locked -p rig-acp --all-targets -- -D warnings on box2: clean
• cargo xtask verify --affected --base origin/main on box2: "All 10 checks passed"
• The test fails on the test-only commit and passes at the head (box2)

Checklist:

• I have updated READMEs and Rust docs affected by this change
• I have added tests that prove my fix is effective or that my feature works
• I did not edit CHANGELOG.md or MIGRATING.md (they are generated at release)

Notes

• Production lines: +10 (plugins/rig-acp/src/router.rs). Test lines: +25.
• Differs from the audit's prior art (f-1.diff): that test asked the shared parser to read the option as a flag or refuse it. This fix refuses in rig-acp, for the reason above.
• rig acp --help is now refused the same way. Before, it was ignored and the server started.
• Not verified: I did not launch rig acp --tools=read from a real editor. The refusal goes through the same refuse_run path (stderr, exit code 2) as rig-acp's other startup refusals.

Follow-ups

• None.