Add a strict exit code and ok flag to dispatch doctor

[jongio]

Problem

dispatch doctor prints a useful setup report, but it always exits 0, even when the config, session store, session-state directory, or Copilot CLI binary is missing. That makes it useless as a gate in an install script or CI step. Right now the only way to check setup programmatically is to run dispatch doctor --json and parse every field yourself.

flowchart TD
    A[dispatch doctor --strict] --> B[Collect doctor report]
    B --> C{Any critical check failing?}
    C -->|no| D[Print report, exit 0]
    C -->|yes| E[Print report, exit non-zero]
    F[dispatch doctor --json] --> G[Report includes ok: true/false]

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Proposed solution

Make the doctor report scriptable without changing the default behavior:

• Add an ok boolean to the --json output that is false when any critical check (config path resolvable, session store present, session-state directory present, Copilot CLI found) is failing.
• Add a --strict flag: dispatch doctor --strict prints the same report but exits non-zero when ok is false. --strict works with --json too.
• Leave plain dispatch doctor and dispatch doctor --json exiting 0 as they do now, so existing usage does not change.

Define "critical" narrowly: a missing or wrong-type config, session store, or session-state path, or a missing Copilot CLI binary. The Copilot CLI version string and session count stay informational and never fail the check on their own.

Acceptance criteria

• dispatch doctor --json includes an ok boolean reflecting whether all critical checks passed.
• dispatch doctor --strict exits non-zero when a critical check fails and 0 when everything passes.
• --strict combines with --json.
• Plain dispatch doctor and dispatch doctor --json still exit 0 regardless of check results.
• Help text documents --strict; completion lists it.
• Unit tests cover the healthy path (ok true, exit 0) and a failing path (ok false, strict exit non-zero).

Notes

collectDoctorReport already gathers every check into a struct, so ok is a small addition. The doctor branch in handleArgs returns an error to main, which already exits 1 on a non-nil error without printing extra text, so wiring the strict exit is straightforward.