docs: add implementation plan for public-activity liveness signal

Single-source spec for v2.1.0 — covers the hard design decisions
(yes/no override, fail-closed, GH_* naming, 30-day API cap, actor-level
bot filtering after the 2025-10-07 PushEvent.commits removal), the
file-by-file code changes, the test matrix, the README structure, and
the known gotchas. Based on round-7 research from five independent
Opus reviewer agents.

Use this as the brief when picking up the v2.1.0 implementation.

Author: oedokumaci