fix: forward leading harness flags when run is the default command

[Copilot]

When run acts as the default command, Commander's positional parsing consumes leading flags like -r and --resume as the optional [agent] argument, producing a misleading "Unknown agent" error instead of forwarding them to Pi.

$ outfitter -r
Unknown agent '-r'. Run 'outfitter list agents' to see resolvable agents.

Changes

• RunAgentCommand.ts — Detect option-shaped agent values (starting with -) in the action handler and reclassify them as pass-through arguments, letting default_agent resolve normally while the flag reaches the harness CLI.
• run-agent.test.ts — Four new tests: -r forwarding, --resume forwarding, explicit agent selection preserved, --harness/--strict preserved alongside forwarding.
• cli.md — Document the default-command pass-through syntax with examples.

Approach

The fix is intentionally narrow — a guard in the action callback rather than restructuring Commander's argument definition. This avoids changing the [agent] positional contract while correctly handling the ambiguity that only arises through default-command dispatch:

if (agent !== undefined && agent.startsWith('-')) {
  effectiveAgent = undefined;
  effectivePassThrough = [agent, ...passThroughArgs];
}

• Fixes Forward Pi flags when run is the default command #239

[ncrmro]

@copilot please rebase this and fix any merge conflicts