docs: update Astro site for v2.0.0 (drop daemon, document RTK parity)

Remove the erroneous "LEGACY (v1.x)" banners that an earlier sweep
injected onto every page — these are the current docs, not legacy.

Bring the site in line with the shipping v2.0.0 binary:
- architecture: process-per-invocation, no daemon/socket/watcher/seal;
  index lives in the XDG cache and auto-builds inline when stale.
- cli: drop removed daemon/warm/hold/projects/query/watch commands;
  add the 33 RTK-parity wrappers, ig import-rtk, the -u/-v/--explain
  global flags, and the rewritten ig setup surface (11 agents).
- vs-rtk: reframe as v2.0.0 RTK feature parity plus the trigram index.
- getting-started / agents: drop daemon setup; document ig setup,
  the 0/1/2/3 hook exit-code protocol, hook-audit, and ig import-rtk.
- performance: drop daemon-only benchmark rows, keep valid numbers.
- embeddings: swap legacy banner for an experimental/opt-in note.

Astro site builds clean (9 pages).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>