feat(rag): decouple indexing from caller timeout via indexing_timeout config (#4073)

[aheritier]

Part of #4073 (T2+T3), depends on #4158 (merged — atomic per-file persistence, T1).

Problem

tools.DefaultStartTimeout (30s) is a shared wait budget for detecting wedged toolsets of any type (#4001). rag.ToolSet.Start honoured the caller's cancellation for manager.Initialize, so when a large knowledge base's indexing outlived the 30s budget, in-flight embedding work (and, pre-#4158, correctness) was discarded and the next turn re-started from scratch.

Fix

Extend the detachment pattern already used for the RAG file watcher (context.WithoutCancel) to manager.Initialize. Indexing now runs on a context derived from the detached watcher context, bounded only by:

• a new indexing_timeout config on RAGConfig (default 30m, 0s = unbounded), and
• the toolset's own Stop.

tools.DefaultStartTimeout's value and the #4062 backoff logic are unchanged — only a clarifying doc comment was added. Raising the shared constant would regress wedged-toolset detection for every toolset type; the fix belongs in RAG's own Start.

#4062 invariant preserved (regression-tested): a detached 429/408/5xx from the model provider during indexing still arms the backoff gate; an indexing_timeout expiry is context.DeadlineExceeded (non-retryable) and never arms it, so the next turn resumes immediately.

Behavior change

Large-KB first turn(s) now proceed without the RAG tool (same UX as any wedged toolset today) instead of blocking up to 30s. Files finished before the budget expires are persisted (per #4158); the tool appears once indexing completes on a later turn.

Changes

• pkg/config/latest/{types,validate}.go: RAGConfig.IndexingTimeout *Duration + GetIndexingTimeout(); negative values rejected, including for top-level rag: blocks (which bypass Toolset.validate() during unmarshal).
• agent-schema.json, examples/rag/semantic_embeddings.yaml, docs/tools/rag/index.md: schema/example/docs for indexing_timeout.
• pkg/tools/builtin/rag/rag.go: WithIndexingTimeout option; Start builds initCtx from the detached watcher context.
• pkg/tools/startable.go: doc-only clarification on DefaultStartTimeout.
• Tests: pkg/tools/builtin/rag/rag_indexing_timeout_test.go (new), rag_backoff_test.go (extended), pkg/config/latest/rag_test.go (new).

Testing

task lint and task test (incl. -race on the rag package) pass clean.