Fix: tech detector excludes vendored/build files only when nested, not at path root (#150)

[sojsun17]

Summary

Fixes _should_skip_file() in agent/tools/tech_detector.py so vendored and
build-output directories are correctly excluded from language detection
regardless of whether they appear at the root of a file path or nested
deeper. The previous implementation matched skip-directories as raw
substrings requiring a leading slash (e.g. "/build/" in filepath), which
missed root-level paths like "build/bundle.js" or "node_modules/x.js"
that have no parent directory — causing repos that are mostly one language
to be misclassified as another based on bundled/vendored files.

Issue

Closes #150

Changes

• Replaced substring matching in _should_skip_file() with path-segment
  matching: split each path on / and check whether any individual
  segment exactly matches a skip-directory name (node_modules, vendor,
  dist, build, .git, __pycache__, .venv, venv).
• This fixes root-level skip-directories while preserving correct behavior
  for nested ones, and avoids false positives on filenames that merely
  contain a skip-dir word as a substring (e.g. src/rebuild/utils.py,
  vendor_utils.py).
• Added 4 new regression tests: root-level node_modules/build exclusion,
  filename-substring false-positive check, and an all-vendored-files case
  that should resolve to "Unknown".
• Added missing assertions to 5 existing tests that called execute() but
  never checked the result (test_vendor_files_excluded,
  test_dockerfile_detection, test_github_actions_detection,
  test_makefile_detection, test_framework_detection).

Testing

• Unit tests pass (make test-unit)
• Integration tests pass (make test-integration)
• Linter passes (make lint)
• Type checker passes (make typecheck)
• New/updated tests cover the changes

Verified the fix directly against the reproduction script from the issue
(reproduce_issue_150.py) — now returns primary_language = Python as
expected, instead of JavaScript.

Pre-existing failures: confirmed via branch comparison — main has 53 failing / 375 passing unit tests before this branch; this branch has 51 failing / 381 passing. All remaining 51 failures are in files unrelated to this issue (test_review_service.py, test_pii_scrubber.py, test_bias_detector.py, etc.) and exist identically on main. This PR fixes 2 test failures and adds 6 newly-passing assertions, all within tests/unit/test_tech_detector.py.

Screenshots / Demo

N/A — backend tool logic change, no UI impact.

Notes for Reviewers

While fixing this, I found two separate, pre-existing bugs in
tech_detector.py that are out of scope for #150 but worth flagging:

1. primary_language selection is alphabetical (sorted(languages)[0]),
   not frequency-based — e.g. a repo with 2 Python files and 1 Dockerfile
   reports "Infrastructure" as primary since it sorts before "Python".
2. Extension-based language detection is actually case-sensitive
   (filepath.endswith(ext) without lowercasing filepath first), despite
   an existing test named test_case_insensitive_extension_matching.

I left both untouched here to keep this PR scoped to #150, but I'm happy
to file a follow-up issue for either/both if that's useful.