refactor(claude): unify subprocess teardown into one idempotent verb (C8)

[TylerLeonhardt]

Stacked on #326552 (C4) → #326270 (C9) → #326256 (C1). Review the stack in order; this PR's diff is one commit.

What

C8 — one teardown path, explicitly idempotent. The pipeline had two teardown spellings that both async-disposed the WarmQuery, and their safety-under-double-call rested on the SDK's close() cleanup happening to be memoized (report 02, H1/H2). C8 collapses them into one private _teardown() guarded by a _torndown flag:

• shutdownAndWait() is the awaited face — used where we must not spawn a fresh subprocess reusing the same --session-id until the old one has released <id>.jsonl (yield-restart / rebuild resume).
• dispose() fires the same _teardown() and forgets. The abort + WarmQuery.asyncDispose run synchronously before dispose() returns, so the subprocess is still signalled to stop promptly.
• Both run abort → WarmQuery.asyncDispose → Query.return in one place, at most once, regardless of which caller wins.
• Replaces the two dispose-chain toDisposables (separate abort + async-dispose) with one.

C9 already deleted the third spelling (_rebindQuery's manual asyncDispose), so this closes out the set.

Why it's safe

• Idempotency is now a property of our code (_torndown), not an SDK implementation detail. A shutdownAndWait followed by dispose (the rebuild path) is a guarded no-op instead of a redundant double-dispose.
• No behavior change to the await-vs-fire-and-forget distinction — shutdownAndWait still awaits actual process exit; dispose still fires-and-forgets.

Validation

• New pipeline test pins the idempotency: shutdownAndWait then dispose async-disposes the WarmQuery exactly once.
• Suites green: claudeSdkPipeline 13, claudePromptQueue 12, claudeAgent 197.
• typecheck-client / eslint / valid-layers-check clean.
• Live smoke suite 19/19 (test/agent-host-e2e/): materialize / warm reuse / recover-rebuild / abort-churn with subprocess reaping — the scenarios that exercise teardown end-to-end against a real subprocess.

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