Bump 0.5.2 -> 0.6.0 and reflect the relevance-gate fail-open contract
change across the docs:
- RELEASE_NOTES.md: v0.6.0 entry (problem, fail-open behavior, the public
contract change, codex 019ef81c clearance, what did not change).
- docs/reference/mcp-tools.md: the quality-gate early-return is now a
fail-open — it refuses only when no source clears the floor.
- skills/research-workflow/SKILL.md: verdict-handling teaches the
fail-open case vs the below-floor refusal.
- CLAUDE.md / AGENTS.md (kept byte-identical): instruction block
distinguishes a fail-open caveat (weakly grounded, relay it) from a
bare refusal (re-gather sources).
- .env.example + docs/reference/configuration.md: document the new
RESEARCH_FAIL_OPEN_MIN_SOURCE_SCORE knob (default 0.3).
No code change in this commit; the gate-demotion implementation landed in
the preceding feat(gate)! commit.