docs: document the concurrency model, convergence guarantees, and non-goals

[joshua-temple]

What

Documents the concurrency behavior shipped in this campaign, and states the deliberate non-goals so users are not surprised.

• internals/architecture.md - a Concurrency and convergence section: per-component concurrency-group isolation (no cross-component cancel); the optimistic read-modify-write with re-apply that all finalize paths use so concurrent component finalizes converge without clobbering siblings; the bounded jittered retry and its cascade-state-write convergence markers; why extra_triggers.merge_group on the side-effecting orchestrate lane is rejected in favor of merge_queue.enabled; and how extra_paths/shared_paths thread a shared change to its consumers.
• Non-goals (same page): no intra-repo component ordering, no per-component state files, no same-component cross-lane serialization - framed as intentional design.
• CONTRIBUTING.md: a breaking generator or validation change (rejecting a previously-valid manifest shape) must scan and migrate the fleet example repos in the same change, since the fleet repin regenerates every example repo with the rc binary before any suite runs.

Verification

Docs site builds (26 pages). go build and go test ./... green. Content verified against the shipped behavior; single-sourced with links. Docs + CONTRIBUTING only, no product code.