fix(core): prune source_tree to changed files under --git-diff-branch

[arimu1]

Problem

With --git-diff-branch, the rendered source_tree still shows the entire repository regardless of how small the diff between the two branches is. This wastes tokens for large repos where only a few files changed — the same problem --include solves for both the tree and file content, but nothing equivalent existed for the branch-diff path.

Fixes #176

Fix

• Added get_git_diff_file_paths() in crates/code2prompt-core/src/git.rs: performs the same tree-to-tree diff as get_git_diff_between_branches(), but collects the relative paths touched by each delta (both sides, so renames contribute old + new path) instead of a text patch.
• Added Code2PromptConfig.diff_files: Option<HashSet<PathBuf>> (configuration.rs) — populated internally, not user-facing.
• Code2PromptSession::load_codebase() (session.rs) now computes this set from config.diff_branches before traversal, so it's available in time to filter the walk.
• discover_files() (path.rs) intersects diff_files with the existing include/exclude match, pruning both source_tree and the collected file content down to just the changed files — mirroring exactly how --include already filters both.

The change is a no-op whenever --git-diff-branch isn't used: config.diff_files stays None and the existing include/exclude behavior is untouched.

Testing

Added a regression test (session_integration_test.rs::test_git_diff_branch_prunes_source_tree_to_changed_files) that builds a two-branch repo where only one of two committed files changes on the feature branch, then asserts the rendered source_tree and file list contain the changed file but not the unchanged one.

Confirmed the test fails without the fix (before adding diff_files/filtering), reproducing the reported bug:

thread 'tests::test_git_diff_branch_prunes_source_tree_to_changed_files' panicked:
expected unchanged file to be pruned from source tree, got:
.tmpJuHtrj
└── src
    ├── changed.rs
    └── unchanged.rs

cargo test --verbose (matching CI) passes for code2prompt-core and code2prompt, including the new test and all existing tests.

Note on prior art

#313 attempted the same fix and was self-closed by its author four months ago with no maintainer feedback (looks abandoned, not rejected). I used it only as a reference for the shape of the change; this PR is a fresh implementation and test against the current main, which has diverged significantly since then.