Add fast_build reasoner with speed-optimized pipeline config

[AbirAbbas]

Summary

• Adds a new fast_build reasoner to swe_af/app.py as a speed-optimized alternative to build(), targeting ~5–10 min turnarounds vs 30+ min for the full build.
• Refactors build() to delegate its core pipeline logic to a private async helper _run_build(), eliminating code duplication.
• fast_build() constructs a BuildConfig with all iteration knobs turned to zero/minimum and delegates to _run_build(), with caller-supplied config still able to override fast defaults.
• All 9 smoke tests in tests/test_fast_build_smoke.py pass, confirming signature parity, fast default values, caller override behavior, and reasoner registration.

Changes

• swe_af/app.py — Core change: extracted _run_build() private async helper; build() now delegates to it unchanged; new @app.reasoner() fast_build() added with identical 10-parameter signature that merges 12 fast defaults before invoking _run_build().
• tests/test_fast_build_smoke.py — New smoke test suite (9 tests) covering AC1–AC9: reasoner registration, signature parity, fast config defaults, caller override, non-reasoner status of _run_build, and file-scope constraint.

Fast defaults applied by fast_build()

Config key	Fast value	Build default
max_review_iterations	0	2
max_retries_per_issue	0	3
max_replans	0	2
enable_replanning	False	True
max_verify_fix_cycles	0	3
max_coding_iterations	1	3
max_advisor_invocations	0	3
enable_issue_advisor	False	True
enable_integration_testing	False	True
agent_max_turns	50	150
agent_timeout_seconds	900	2700
git_init_max_retries	1	3

Test plan

• pytest tests/test_fast_build_smoke.py -v — all 9 tests should pass
• Verify fast_build appears in [r['id'] for r in app.reasoners]
• Smoke-run fast_build on a trivial repo and confirm completion in <10 min
• Confirm build() behavior is unchanged (full pipeline, default BuildConfig)
• Confirm caller-supplied config dict overrides fast defaults in fast_build()

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