feat(cli): add agk doctor environment diagnostics

[kunalkushwaha]

What

Adds a preflight agk doctor command that surfaces common first-run failures before an agent runs, instead of letting them appear as cryptic runtime errors deep inside the framework.

This is feature B3 from the FEATURES.md roadmap.

Checks

Check	OK	Warn	Error
Go toolchain	go present, ≥ go1.24	—	missing / too old
LLM credentials	a cloud key is set	no cloud keys set	(with --provider X) X's key missing
Ollama	reachable + model count	not reachable	—
Template registry	reachable + template count	unreachable	—
Project workspace	go.mod found (+ trace run count)	not in a project dir	—

Exits non-zero if any error-level check fails, so it can gate a CI preflight step. --provider openai|anthropic|azure|ollama makes that provider's credential a hard requirement.

Example

🩺 AGK Doctor — environment diagnostics
  ✓ Go toolchain             go1.26.4
  ⚠ LLM credentials          no cloud provider keys set
    ↳ export OPENAI_API_KEY / ANTHROPIC_API_KEY, or use Ollama locally
  ✓ Ollama                   running at http://localhost:11434 (6 model(s))
  ✓ Template registry        reachable (3 template(s))
  ✓ Project                  go.mod found; 0 trace run(s) in .agk/runs

⚠ 1 warning(s) — you can still build and run

Testing

• go build, go vet, go test ./... green; gofmt clean.
• Verified live: default run (Ollama detected with real model count, registry reachable), --provider openai with key unset (→ error, exit 1), key configured (→ OK), and outside a project dir (→ workspace warning).

Note on branch

Branched from main, so the roadmap docs (FEATURES.md/CLAUDE.md, introduced in #13) aren't in this PR. I'll mark B3 as shipped in FEATURES.md once #13 lands. There will be a trivial commands-table conflict with #13 — straightforward to resolve.

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