fix: propagate context cancellation to background jobs and handle interrupted waits

[lockp111]

What

Background shell jobs were launched with context.Background(), making them immune to Esc (agent cancellation) and session shutdown. This left zombie processes that couldn't be cleaned up without manual job_kill.

Additionally, job_output with wait=true silently discarded the WaitContext return value, so context cancellation during a wait was invisible — the tool returned stale output as if nothing happened.

Changes

internal/agent/tools/bash.go

• Both run_in_background and synchronous auto-background paths now use the agent's request context (ctx) instead of context.Background() when calling bgManager.Start.
• When the agent is canceled (Esc or session shutdown), the context cancellation propagates through shellCtx → exec.CommandContext → process killed → goroutine exits → done channel closes.

internal/agent/tools/job_output.go

• WaitContext return value is now checked. When it returns false (context canceled), the tool returns an "interrupted" status with the current partial output, instead of silently falling through to the normal path.

internal/agent/tools/job_test.go

• Added 3 new tests:
  • TestJobOutput_WaitContextCanceled — canceled context returns "interrupted"
  • TestJobOutput_WaitCompletes — normal completion returns "completed"
  • TestJobOutput_NoWaitReturnsRunning — no-wait path returns immediately with "running"

Why

• Background jobs (servers, long builds) started by the agent now get properly cleaned up on Esc
• job_output wait path no longer silently ignores context cancellation
• No API changes, no new dependencies
• All existing tests pass unchanged

💘 Generated with Crush

Assisted-by: Crush:deepseek-v4-pro

[charmcli]

All contributors have signed the CLA ✍️ ✅
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[lockp111]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.

[lockp111]

@meowgorithm @andreynering who can take a look at this MR?