docs(governance): publish sustainability and continuity posture

[tcballard]

Summary

Closes DEP-11 from the enterprise deployability review.

• add a root project-governance statement that names the solo-maintainer model without inventing a board or foundation
• state the best-effort support, current-stable-only support, and demand-led release posture
• document practical continuity through Apache-2.0, the public spec and conformance fixtures, local git-owned corpora, no hosted dependency, and rebuildable caches
• distinguish those properties from formal source escrow or a commercial support agreement
• rename the existing docs navigation/page from Governance to Enforcement Policy and cross-link the real project governance policy
• pin the public claims in the native docs-contract suite

Validation

• git diff --check
• rustfmt --edition 2021 --check decided-mcp/tests/docs_contract.rs
• cargo test -p decided-mcp --test docs_contract --release (3 passed)

Notes

The repository-wide cargo fmt --all --check reports pre-existing formatting drift across untouched Rust files; the changed Rust test itself passes rustfmt --check.