👌 Replace gitignore-parser with ignore-python and add per-project follow_links

[Copilot]

Replaces gitignore-parser with ignore-python (Rust ignore crate bindings) for file discovery.
This provides native nested .gitignore support, improved performance,
and behavioral parity with ubcode.
Adds per-project follow_links config.

Dependency swap

• gitignore-parser>=0.1.11 → ignore-python>=0.3.3

Source discovery rewrite (source_discover/source_discover.py)

• WalkBuilder replaces Path.rglob() + gitignore_parser.parse_gitignore() + fnmatch
• Include/exclude patterns mapped to OverrideBuilder globs
• File type filtering stays in Python (overrides would take precedence over gitignore rules)
• Non-existent source directories now return empty list (WalkBuilder raises; old rglob silently yielded nothing)

Walker filter alignment with ubc_codelinks

The walker configuration now replicates the Rust ignore crate's
standard_filters(gitignore) followed by hidden(false),
matching ubcode
behaviour exactly. Since the Python ignore-python bindings don't expose
standard_filters(), we set all six individual filter methods:

builder.ignore(gitignore)     # .ignore file support
builder.parents(gitignore)    # parent ignore files
builder.git_ignore(gitignore) # .gitignore
builder.git_global(gitignore) # global gitignore
builder.git_exclude(gitignore)# .git/info/exclude
builder.hidden(False)         # always show hidden files

Previously, git_global and git_exclude were hardcoded to False and
.ignore/parent ignore support was not configured.

New follow_links config

• follow_links: bool = False added to SourceDiscoverConfig, both TypedDicts, CLI discover command, and src_trace directive passthrough

Semantic change: include/exclude

With the ignore crate's override system, include whitelists files (overriding gitignore),
then exclude removes from that set.
Previously include had absolute priority over both gitignore and exclude.

[codelinks.projects.myproject.source_discover]
src_dir = "./src"
follow_links = true
gitignore = true
include = ["**/*.cpp"]
exclude = ["build/*"]

Shared portable test fixture

Added a shared JSON test fixture (tests/data/discover_fixtures.json)
with 18 test cases covering all discovery behaviours.
The same fixture is consumed by both this project and ubc_codelinks
to verify matching behaviour across the Python and Rust implementations.

Test cases cover:

• Basic file extension discovery
• .gitignore (root-level and nested)
• .ignore file support
• include and exclude patterns (individually and combined)
• Default include derived from language extensions
• Hidden file discovery
• All comment types (cpp, python, rust, cs, yaml)
• Non-existent src_dir edge case
• Deeply nested directory trees
• All C++ file extensions

Documentation updates

• configuration.rst: Added follow_links option docs, expanded gitignore section to list all supported ignore sources (nested .gitignore, .ignore, .git/info/exclude, global gitignore, parent ignore files), removed "Nested .gitignore is NOT supported" limitation note, corrected include/exclude priority description
• discover.rst: Added follow_links to advanced filtering example
• roadmap.rst: Marked nested .gitignore support as completed

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.04%. Comparing base (a2a79e4) to head (8fbce16).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
+ Coverage   89.80%   90.04%   +0.23%     
==========================================
  Files          29       29              
  Lines        2560     2621      +61     
  Branches      300      306       +6     
==========================================
+ Hits         2299     2360      +61     
  Misses        165      165              
  Partials       96       96              

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