docs: qualify README API coverage and raw access claims

[joshrotenberg]

Scope

README-only contribution to #1085; leaves #1085 open.

Replaced five absolute/universal claims with descriptive Cloud/Enterprise/MCP/database capability language. Added an explicit CLI-vs-MCP/product-version qualifier, linked the unfinished support/validation matrix, and clarified that raw API requests remain subject to upstream endpoint availability, payloads and permissions.

No full matrix or new supported/tested version/architecture claims. No runtime, defaults, credentials, configuration schema, CI/settings policy or release changes. Other open PRs remain untouched.

Validation

• Focused static assertion: all five identified absolute claims removed, required qualifiers present, and every existing fenced example unchanged against main.
• Whole-file Markdown lint still has exactly the five baseline diagnostics (MD060 x4, MD032 x1); normalized diagnostics match and no suppression added.
• New docs(1.0): publish a support matrix and evidence-based coverage claims #1085 link returned HTTP 200. Repository search found no references to the renamed heading anchors; external bookmarks cannot be inventoried.
• Diff check and required formatting/all-target/all-feature Clippy passed before push (cold kickoff Clippy 1m02s; final 0.77s).
• Independent read-only agent review at the exact pushed head found no actionable findings. Prose matches the current compatibility policy, CLI parser and raw request dispatch. Review did not claim live API coverage or run broad suites.
• Local unit/integration/strict site suites were not repeated for this README-only change. Hosted exact-head CI is complete: 15 successful checks and seven expected PR-only/main-only skips. Required unit/integration/Linux validation passed; macOS/Windows builds, strict MkDocs, link-check and Rustdoc jobs also passed. Markdown lint remains advisory; the five local baseline diagnostics are still tracked as existing debt.

Checkpoint

Head: bc3c40b80b70e5bbdd61c0e40c29ef5c08bb4e27.
Implementation, focused local validation and independent review finished. Exact-head hosted checks reconciled (including individual mandatory jobs/steps, not just aggregate status); ready for required human review. No merge/release. The full #1085 support matrix remains outstanding.

CI · Documentation · Release plan.