feat(project): wire dev handler

[tejaskash]

Summary

• implement agentcore project dev runtime selection, port resolution, runner dispatch, structured output, and signal handling
• reuse withProject for project discovery and actionable missing-project guidance
• load runtime and .env.local values directly; containers forward host AWS variables, mount ~/.aws read-only, and fail early when neither credential source exists
• reuse shared JSON rendering for NDJSON events and generalize command interruption handling

Stacks on #1962.

Verification

• bun test (1107 pass)
• bun run typecheck
• bun run lint:check
• bun run format:check
• bun run build
• executable smoke: scaffolded a real Python project, ran project dev, received healthy /ping, interrupted with exit 130, and verified child cleanup and port release
• executable no-project probe verified the actionable project create guidance

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.00990% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.06%. Comparing base (2dba459) to head (f0b65c9).

Files with missing lines	Patch %	Lines
src/core/dev/path.ts	92.59%	2 Missing ⚠️
src/handlers/project/dev/environment.ts	97.14%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           refactor    #1966      +/-   ##
============================================
+ Coverage     97.04%   97.06%   +0.02%     
============================================
  Files           370      374       +4     
  Lines         22287    22542     +255     
============================================
+ Hits          21628    21880     +252     
- Misses          659      662       +3     

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[Hweinstock]

only blocking comment is the direct process kill.

[aidandaly24]

This LGTM, I will replace the user cancellation error with mine in #1986

[aidandaly24]

Saw that Harrison had also approved earlier so merged