Add a patch-safe file update tool for large files

## Problem

`create_or_update_file` and `push_files` require the MCP client/model to send the complete replacement file content. For large files this can exceed host/request limits before the GitHub API receives the intended bytes.

A real private-repository workflow hit this with a ~303 KB changelog: a one-entry append required resending the entire file, the MCP request was truncated upstream of GitHub, and the resulting commit contained only ~70 KB. The bad PR was detected and closed, but a local Git client was required to recover the exact patch.

## Proposed capability

Add a narrowly scoped patch-safe update tool, e.g. `apply_file_patch` or `create_commit_from_patch`, that fetches the current file server-side and applies a bounded exact patch without requiring the MCP client to transmit the entire replacement file.

A conservative initial contract could use exact text replacements rather than fuzzy patching:

- `owner`, `repo`, `branch`, `path`
- `expected_head_sha`
- `expected_blob_sha`
- ordered edits containing exact `old_text` and `new_text`
- each edit must match exactly once (or an explicitly supplied exact occurrence count)
- `commit_message`

Required behavior:

1. Read current branch/head and fail if `expected_head_sha` differs.
2. Fetch the current blob/file server-side and fail if `expected_blob_sha` differs.
3. Apply edits exactly; no fuzzy offsets or best-effort matching.
4. Preserve file mode/path and reject binary/oversized/ambiguous inputs for the initial implementation.
5. Commit only after all edits and validations pass.
6. Return before/after blob SHA, commit SHA, tree SHA, changed path, and size.
7. Never force-update a ref; concurrent branch movement must fail closed.

A later version could support a bounded unified-diff parser and atomic multi-file commits through the Git database APIs.

## Why this belongs in the server

The server already has the authenticated GitHub client and can fetch the full current blob directly. Keeping the full file server-side avoids model/context/request amplification and materially reduces the risk of truncation while retaining optimistic concurrency checks.

## Scope / non-goals

- No arbitrary shell or local Git execution.
- No fuzzy patch application.
- No force push.
- No bypass of repository permissions or branch protections.
- Keep the tool outside read-only mode and subject to the same OAuth/repository scope filtering as existing content-write tools.

I can prepare a focused PR with tests if this shape is acceptable.

Start by tracing the existing create_or_update_file and push_files tools and their GitHub content-write paths. Define the proposed patch-safe tool around the stated optimistic-concurrency, exact-edit, validation, and fail-closed requirements, then add focused tests covering those behaviors and the returned commit metadata.

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