v0.2.0 - native drift detection

bairn drift is now a native Go subcommand. Loads a TOML manifest,
hits each endpoint with auth + paced delay, computes JSON-key-only
shape signatures, optionally diffs against a prior baseline.
Surfaces vendor schema changes before they break a fetch.

Exit codes: 0 = no drift, 1 = drift found, 2 = config or transport
error. Wires cleanly into the claude-drift-triage component in
dunn.dev/pipeline so the LLM triage fires only on real changes.

The Python prototype (discovery/probe/shape.py) stays in the repo
as documentation of the methodology. Signature format is
byte-compatible.

Posture and conditions of use unchanged: NOTICE.md.