docs: rename to UPPER_SNAKE convention + drop superseded mars-genesis v2

Two cleanups in docs/:

- Rename key-rotation.md to KEY_ROTATION.md to match the all-caps naming
  convention used by every other top-level doc (README, INGEST_ROUTER,
  MEMORY_ROUTER, READ_ROUTER, READER_ROUTER, COGNITIVE_PIPELINE, etc.).
- Consolidate the mars-genesis spec docs:
  * delete mars-genesis-v2-design.md (superseded by v3)
  * delete mars-genesis-v2-recommendations-2026-04-12.md (Codex review of
    the v2 spec; rolled into the v3 design)
  * rename mars-genesis-v3-design.md -> MARS_GENESIS.md (with v2 gone the
    version suffix is just noise; this is the active spec)

The companion plan files (mars-genesis-v2-plan-phase1, -phase2,
mars-genesis-v3-plan) are already gitignored and remain as local drafts.
Neither key-rotation nor any mars-genesis doc is in the publication
manifest, so docs.agentos.sh is unaffected. Internal cross-references
in MARS_GENESIS.md and KEY_ROTATION.md don't point at any of the
removed files.

Author: jddunn