docs(mcp): define fixed-target migration contract and pilot gates

[joshrotenberg]

Scope and checkpoint

• design(mcp): define the database target contract for redis-mcp adoption #1171: proposed fixed-target startup contract, explicit URL/profile conflict and ambiguity rules, credential boundary, multi-target pattern and 1.0 compatibility treatment.
• feat(mcp): add a feature-gated redis-mcp read-only pilot #1172: pilot parity/catalog acceptance matrix and development/release gates; no runtime implementation in this PR.
• Leaves epic(mcp): migrate Redis database tools to redis-mcp #1170, design(mcp): define the database target contract for redis-mcp adoption #1171 and feat(mcp): add a feature-gated redis-mcp read-only pilot #1172 open. Human contract acceptance is still required.

Refreshed against main 6837dc1e3e7b3aaf35e741bb382fe03218fd1563 (merged skills #1174), without rewriting history. Current tested/pushed head: a8ed63b9c7f151b08b4012540bae97060cd93840.

Result

One explicitly selected database target per process; no per-call target fields, mutable target switching or silent fallback. Legacy remains default. This documentation-only slice changes no runtime/schema/dependency.

Current status now links the isolated pilot #1176, immutable library pin 8eff8240679d38157ee08b9d5e5eaeb656ce7550, merged Tower baseline/hardening dependency, embedded skills catalog composition and the remaining library setup #128/lifecycle #125 gates. The document distinguishes local/Git development from later publication/registry packaging; publication is not a pilot prerequisite.

Source-access guidance inventories all Cargo-resolving workflows, including rustdoc, cargo-deny and dist metadata. Trusted-CI credentials alone would not solve anonymous/default-feature or public-fork consumption of an optional INTERNAL Git dependency. No credentials, visibility change, release-token reuse, fork-secret exposure, check bypass, release or default cutover is authorized.

Current-head local validation — 2026-10-05

• Strict MkDocs build and focused Markdown lint: pass; merged skills page and migration navigation compose without conflicts.
• git diff --check; cargo fmt --all -- --check: pass.
• cargo clippy --all-targets --all-features -- -D warnings: pass.
• cargo test -p redisctl-mcp --all-features --lib --test skill_resources --test skill_stdio: 167 + 9 + 1 pass.
• Strict all-feature workspace rustdoc: pass.

At 10:31:46 PDT, current-head CI quick checks, all three unit suites, integration tests, Markdown/MkDocs/link checks, strict rustdoc and dist plan are green. Platform builds remain pending; the overall CI gate is not yet reported green. This PR is ready for human review, not merged. The private dependency is only in pilot #1176, not this contract branch. Contract implementation/parity evidence belongs to #1176; these local tests are not a claim that the full migration or proposed compatibility decision is accepted.