fix: parse Droid settings.json as JSONC during hook install

[rajaiswal]

Summary

• Fix Droid hook installation failing when ~/.factory/settings.json contains JSONC comments
• Parse Droid settings with jsonc-parser (already a project dependency) instead of strict serde_json
• Add tests for JSONC settings with line comments, block comments, and trailing commas

Test plan

• task test TEST_FILTER=droid
• task fmt and task lint
• Manually verify git-ai install succeeds for Droid on a machine with JSONC settings

Fixes Droid hook install error: JSON error: expected value at line 1 column 1

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[CLAassistant]

All committers have signed the CLA.

[devin-ai-integration]

Devin Review found 1 new potential issue.

[devin-ai-integration]

🚩 generate_diff shows comment removal alongside hook changes

In both install_hooks_at (line 267) and uninstall_hooks_at (line 329), generate_diff compares the original raw JSONC content against the new pretty-printed JSON. This means the diff shown to users will include not just the hook modifications but also the removal of all JSONC comments and any formatting changes. This could be confusing to users who see a large diff when only a small hook change was intended. The VS Code settings equivalent in src/mdm/utils.rs:784 avoids this by using CstRootNode which preserves formatting.

(Refers to lines 266-267)

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