🤖 fix: avoid backgrounding exited bash runs

[ammar-agent]

Summary

• avoid backgrounding already-exited bash runs by draining foreground output
• return an immediate error for mux.md URLs missing a key to avoid network fetches
• retry the executeBash env check in Windows integration tests to reduce flake
• capture the deterministic coordination guidance in AGENTS.md

Why

Fast-exit commands were being backgrounded after they finished, returning empty "Output so far" and flaking Windows CI. mux.md URLs without a key were also falling through to network fetches, causing the unit test to time out. The updates make both flows deterministic and resilient.

Testing

• make static-check
• bun test src/node/services/tools/web_fetch.test.ts -t "should return error for mux.md URLs without valid key format"

Regression risk

Low. The bash tool change only affects the fast-exit/foreground-to-background transition, with outputs drained before any background migration. The mux.md validation now short-circuits invalid share URLs rather than attempting a network fetch. The Windows test retry is scoped to the integration test helper.

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