Merge BuildAdapterFilesystem and src/index/adapter.h

[jviotti]

Signed-off-by: Juan Cruz Viotti jv@jviotti.com

[cubic-dev-ai]

No issues found across 8 files

[augmentcode]

🤖 Augment PR Summary

Summary: This PR merges the index-specific filesystem adapter into the shared build adapter.

Changes:

• Refactors BuildAdapterFilesystem to be constructed with a root directory and to resolve/write dependency paths relative to that root
• Standardizes dependency files to use a fixed .deps extension (removing custom extension support)
• Removes src/index/adapter.h and updates the index binary to use BuildAdapterFilesystem directly
• Updates unit/e2e tests to pass an explicit root directory when constructing the adapter

Technical Notes: Relative dependency entries are now expanded with weakly_canonical(root / dep) when reading, while dependencies under the root are persisted as relative paths when writing.

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[augmentcode]

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