feat(cli): add 'record' subcommand — capture traces from any callable (E3) - #23

What

agentdiff record module:function --input '{...}' --out trace.json — runs any callable once and captures a canonical AgentDiff trace. Kills the biggest onboarding wall: "how do I get a trace?"

Why

E3 from the release plan (0.4.0 Capture, Fri 2026-08-29). Visitors bounced at setup because they had no trace to diff. record makes the install → record → diff → gate loop work in <5 minutes with zero telemetry.

How

• recorder.py: resolves module:function / module:Class.method (imports from CWD, pytest-style), times execution, captures return/errors as a canonical AgentTrace (single step, Generic format)
• Failed runs are still recorded (exit 1) — diff a failure to see exactly what broke
• E1 backward compat preserved: agentdiff base.json cand.json (no subcommand) still means diff, via a click.Group parse_args patch that works in both the real CLI and CliRunner
• 19 new tests (319 total green), cookbooks/record_callable.py demo, README + quickstart updated

Testing

• make lint clean · pytest 319 passed · pnpm run build clean
• Manual end-to-end: record baseline → record changed candidate → diff blocks gate (exit 1); failed-run recording; input from @file

Checklist

• Branch + one logical change
• Lint + tests green
• Docs (README quickstart §0, docs quickstart §2, cookbook + index)