docs: fix FEDERATION_V2.md's inverted staleness

[wikithoughts]

What

docs/FEDERATION_V2.md had drifted stale in the inverted direction: its header and per-section
statuses said "design"/"roadmap" for work that actually shipped months ago, per the settled ADRs in
docs/DECISIONS.md. This PR brings the doc's statuses back in line with reality — no code changes.

Changes

• Header: now states steps 0–3 are built, step 4 is partially built, §5 is built (was "design").
• §0 assisted task class: — BUILT (D21, R9). Build phases (a) and (b) marked done; phase (c)
  — explicit licensed-software/heavy-compute capability-declaration flags at node registration —
  confirmed genuinely not built (verified via grep -rn "licensed-software|heavy-compute" council/,
  no hits) and left open.
• §3 content-addressed artifact store: BUILT (D22, R10). Rewritten present-tense to describe
  what shipped: 256 KiB chunking, sha256 content-addressing, Merkle-root verification, pw deliver ... @file, pw fetch (per council/artifacts.py).
• §4 consent + capability disclosure: PARTIALLY BUILT — links D53 (class-level pw join
  consent + per-task assisted consent) and D24 (reputation/capability gating via store._meets /
  store._meets_reputation in council/net/store.py), both settled; the declared-capability-flags
  gap (same as §0 phase c) is the one remaining open item.
• §5 MCP interop: BUILT (D19, R7), not D18 — rewritten present-tense: pw mcp, the [mcp]
  extra, council/mcp_server.py, 3 exposed tools (research, library_search, library_add).
• Prefers linking D-numbers in docs/DECISIONS.md over restating shipped detail.

Verification

• ruff check . — clean.
• pytest tests/ -q — 507 passed, 1 skipped (docs-only change; this is a sanity check the worktree
  is otherwise healthy). Note: an earlier full-suite run under heavy shared-host CPU contention
  showed 2 flaky failures in tests/test_agent_loop.py (timing-sensitive real-coordinator-thread
  tests); both pass individually in <1s and pass again on a clean full-suite rerun once the shared
  box's load settled — confirmed unrelated to this docs-only change.

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