Prepare agentctl for public release

[nzrbits]

What & why

Clean up agentctl so the repo can go public: consistent defaults, clearer docs, license and CI.

Changes

• Default caller is now local in code and example config, matching the README. Unused config keys removed.
• collect_stream_text and extract_json are public helpers; small import and type cleanups; status labels ChatGPT handoff runs.
• Removed the unused claude-builder prompt.
• New README with design rationale, docs/reliability.md for upstream quirks, updated .agent/README.md and AGENTS.md.
• MIT license and a GitHub Actions workflow running the tests on macOS (Python 3.11, 3.12).

Verification

• 39 unit tests pass locally on Python 3.11, 3.12 and 3.14.
• 3 of the 5 new config tests fail on the old code (negative control).
• History scanned for secrets, private paths and employer data: none found.

Open points

None