Stop the stall watchdog from aborting healthy parallel tool fan-outs

[TheGreatAxios]

Summary

• awaitingResponse flips true the instant any one tool call in a parallel fan-out finishes, even while sibling calls are still running
• The stall watchdog treated that flag alone as proof of silence, so a healthy multi-agent run got a false stall notice at 90s and a hard abort at 900s, killing every live sub-agent
• Threaded activeToolCalls into ShouldAbortForStallArgs so outstanding tool calls suppress both the notice and the abort; genuine stalls (no active calls) still abort as before

Test plan

• Added a failing test first: active tool call + awaitingResponse: true + 20-minute-stale lastActivityAt -> shouldAbortForStall false, shouldNoticeStall false; confirmed it failed before the fix
• Existing genuine-stall tests (no active calls) still pass
• bun run typecheck — only pre-existing, unrelated vendor/intx-inference/arktype errors
• bun run build — passes
• bun run test — 4080 pass, 1 pre-existing environment-dependent failure (lsp-availability.test.ts, unrelated to this change)

[linear-code]

CL-5641

[TheGreatAxios]

Root-cause follow-up filed per Greybeard's review: CL-5661 (https://linear.app/abklabs/issue/CL-5661) — turn-state.ts's tool.done handler sets awaitingResponse unconditionally instead of gating on remaining activeToolCalls like inference.done/connector.reply already do. This PR's fix at the watchdog layer is correct and matches the Linear acceptance criteria; CL-5661 tracks fixing the underlying asymmetry at the source.