docs: bring docs up to date for Horizon 2.10 (tool counts, versions, capabilities)

[souf92i]

Audited all tracked product/dev docs against the merged 2.10 config-CRUD surface and fixed the stale references.

README.md

• per-domain table: add Configuration (126), Assist 20 -> 21; sums to 211 / 12 domains
• prose: 'The 84 tools ... 11 domains' -> 211 / 12; 'All 84 tools' -> 211
• prerequisites + Compatibility table: add 2.10 as the primary tested QA target (was 'tested on 2.8'); 2.9 grouped with 2.7 as expected-to-work
• 'What is not supported' rewritten: it wrongly listed CAs, profiles, connectors, roles/teams, scheduler, system config, etc. as unsupported - these now have CRUD. Now lists only the genuine read-only surfaces (identity providers, service accounts, grading policies/rulesets, credentials) and out-of-scope items.

docs/client-setup.md - '84 tools' -> '211 tools' (x2)
docs/installation.md - 'tested on 2.8 ...' -> 2.10 primary
docs/tools-reference.md - Configuration 'System & operations' 20 -> 25 so the sub-sections sum to 126

Docs-only; no code changes. Found via a 15-agent read-only audit (47 findings); the remaining items (COVERAGE.md, a design doc's status line, a few knowledge-doc examples, and gitignored CLAUDE.md/skills files) are noted separately for follow-up.