Fix vendored/build path exclusion in TechDetector (closes #150)

[DataMnk]

Summary

Fixes the TechDetector tool so it correctly excludes vendored/build files
(e.g. node_modules/, build/) when detecting a repository's primary
language, instead of incorrectly counting them and skewing the result
toward JavaScript.

Issue

Closes #150

Changes

• Rewrote _should_skip_file() in agent/tools/tech_detector.py: instead
  of substring-matching slash-anchored patterns like "/node_modules/"
  (which silently failed to match relative paths without a leading slash,
  e.g. "node_modules/lib/index.js"), it now splits each filepath into
  path segments and checks whether any directory segment (excluding the
  filename itself) exactly matches a known vendor/build directory name
• This correctly excludes paths with or without a leading slash, while
  avoiding false positives on filenames that merely contain a vendor
  directory name as a substring (e.g. src/node_modules_helper.py)

Testing

• Unit tests pass (make test-unit)
• Integration tests pass (make test-integration)
• Linter passes (make lint) — verified on the modified file with
  ruff check agent/tools/tech_detector.py
• Type checker passes (make typecheck)
• New/updated tests cover the changes — the two previously-failing
  tests named in the issue, test_node_modules_excluded and
  test_build_directory_excluded, now pass. I also added three new
  regression tests for edge cases identified in my PLAN.md: a
  lookalike filename that should NOT be excluded
  (test_node_modules_lookalike_filename_not_excluded), a build/vendor.js
  path (test_build_vendor_file_excluded), and Windows backslash paths
  (test_windows_backslash_paths_excluded). Full file suite is now 30/30

To verify by hand:

1. git checkout fix/150-tech-detector-vendored-files
2. pytest tests/unit/test_tech_detector.py -v — all 30 tests pass
3. In a Python REPL:

   from agent.tools.tech_detector import TechDetector
   files = ['main.py', 'node_modules/lib/index.js', 'build/bundle.js']
   TechDetector().execute({'files': files}).data['primary_language']
   # Returns 'Python' instead of 'JavaScript'

Screenshots / Demo

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

Notes for Reviewers

When I first chose this bug, it took me some time to understand what it
was really about, so I started exploring the repo structure and files.
That took a while but there was a lot of new stuff to take in, so the
time was worth it. Once you understand it, it's pretty intuitive.

I used Claude to help explain the existing code and propose some fixes,
and this approach (splitting into path segments) was the one that made
the most sense to me — the best option out of what we considered.

There's always a lot to learn along the way, and it was tempting to look
into other bugs I noticed while exploring. That would have led to me not
delivering anything and just exploring endlessly, so I forced myself to
focus and just work on my bug.