Add a doctor health-check for environment and setup #5

Summary

Add a self-diagnostic doctor health-check that verifies a user's environment and Kagan
setup and prints clear pass / warn / fail results with remediation. Today the onboarding flow
assumes a working environment; when something is off (wrong OpenCode version, plugin not
registered, missing provider auth, un-writable worktree base) the failure surfaces late and
opaquely as a broken board or a helper that never starts.

Proposal

A diagnostic that runs a set of independent checks and reports each with a status and an
actionable next step. Runnable standalone and suitable for embedding in onboarding.

Candidate checks:

• Bun present and a supported version.
• OpenCode version within the manifest engines.opencode range (>=1.17.13 <1.18.0).
• Kagan is registered in the project opencode.json plugin array.
• git is available and the repo has a work tree.
• The worktree base directory is writable (worktrees are created there).
• At least one model provider is authenticated.

Design notes

• Uniform contract so the runner/formatter stay generic: each check is a
  { id, name, category, run(): Promise<Result> }; a Result is
  { status: "pass" | "warn" | "fail", message, details?, remediation? }.
• Runner isolates each check in try/catch so one throwing check cannot abort the run; it
  tallies results and exits non-zero if any check fails.
• Human output plus a --json mode; failing checks carry a concrete remediation line
  (e.g. the exact command to run).
• Test each check as a pure map from gathered inputs to a Result (split the impure
  "gather" from the pure "classify"), so checks are unit-testable without hitting the
  network or filesystem.

Acceptance criteria

• Running the doctor on a healthy setup reports all pass and exits 0.
• Each failable condition has a check that returns fail with a remediation line, and a
  unit test covering its pass / warn / fail branches.
• One throwing check does not crash the run.