feat: add --test-all flag to force testing all packages

[vanandrew]

Summary

• Adds --test-all CLI flag that skips git diff entirely and reports all workspace packages as affected
• Exposes test-all input on the GitHub composite action
• When --test-all is set, no git operations are performed — only the lock file is parsed to enumerate packages
• --exclude still works with --test-all to filter out specific packages

Usage

CLI:

difftrace --test-all --lock-file uv.lock --json

Action:

- uses: vanandrew/difftrace@main
  with:
    test-all: "true"

Test plan

• Parser test: --test-all flag parses and defaults to False
• --test-all returns all packages without calling git
• --test-all combined with --exclude filters correctly
• --test-all excludes virtual root packages
• Existing tests unaffected (126 passing)
• Lint, format, and type checks pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.38%. Comparing base (a1a6363) to head (88bf136).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
+ Coverage   96.31%   96.38%   +0.06%     
==========================================
  Files           6        6              
  Lines         326      332       +6     
==========================================
+ Hits          314      320       +6     
  Misses         12       12              

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