bazel: sub-agent roles, contracts, and orchestration for #1 #13

Context

Issue #1 is large enough that M2 delivers it via sub-agents. This issue defines roles, contracts, handoffs, and ownership so parallel work does not drift from #1's build-system contract.

Work

• Document sub-agent roles (examples: inventory/baseline, Bazel bootstrap, library modeling, test-graph modeling, bindings packaging, parity/evidence, cache/perf, cutover, docs/security).
• Define each role's inputs, outputs, non-goals, and which build: migrate CI builds and tests to Bazel with Blacksmith caching #1 AC / implementation-sequence step it owns.
• Define handoff artifacts (migration ledger updates, target maps, parity notes, benchmark evidence paths).
• Define conflict rules: no Cargo-shell Bazel targets; no silent Cargo↔Bazel feature drift; no secrets in cacheable actions; keep CI Gate name during dual-build.
• Point agents at build: migrate CI builds and tests to Bazel with Blacksmith caching #1 as the specification; children bazel: freeze migration inventory and Cargo/Blacksmith baseline #12–bazel: documentation, observability, and #1 close-readiness evidence #3 as the execution slices.

Acceptance criteria

Given issue #1 as the canonical specification
When sub-agent roles and contracts are checked into the repo or attached as durable issue/PR evidence
Then every M2 Bazel child (#12–#3) maps to a named role and #1 sequence step
And mobile bindings (Swift/Kotlin/UniFFI) are explicitly excluded from all role charters
And handoff artifacts required by later slices are named

Out of scope

• Implementing Bazel targets (owned by bazel: Bazelisk, Bzlmod, rules_rust bootstrap and drift checks #11–bazel: model PyO3 and napi-rs cdylibs plus packaging handoff #7).
• Performance measurement (bazel: Blacksmith remote cache enablement and cold/warm performance gates #5) or CI Gate cutover (bazel: cut over CI Gate, retain rollback, retire Cargo sticky disks #4).
• Mobile bindings work of any kind.

Evidence

• Durable orchestration note (docs path and/or PR) referenced from this issue.
• Ordinary AGENTS.md close.

Relationships

• Parent / blocks: M2 gate: implement #1 Bazel migration via sub-agents #2
• Blocks: bazel: freeze migration inventory and Cargo/Blacksmith baseline #12 (inventory freeze uses the contract)
• Canonical spec: build: migrate CI builds and tests to Bazel with Blacksmith caching #1