feat: add file mode (executable bit) support to push_files

[why-pengo]

This is my first contribution to this repo. I used Claude Code with Opus 4.7. I reviewed the code and tried to make sure I followed contributing.md.

Summary

• Adds an optional mode field to each entry in push_files, threaded into the Git Data API CreateTree call. Accepted values are the ones the API itself accepts: 100644 (regular file, the default when omitted), 100755 (executable), 120000 (symlink), 040000 (subtree), 160000 (submodule). Unknown values are rejected with a clear error.
• Updates create_or_update_file's description to point callers at push_files (with a single-entry files array) when they need to land an executable file, since the Contents API can't set the mode.
• Backwards compatible: existing callers that omit mode continue to write 100644.

Motivation

Agentic harnesses use this server to publish files into a repo on behalf of an LLM. When the produced artifact is a shell script, build helper, or other executable, the file needs the executable bit set — otherwise reviewers either have to chmod +x after merge or land a broken artifact. Today there is no in-band way to do this through the server.

In practice we observed two failure modes:

1. Agents reporting in PR bodies that they pushed with mode 100755 when the blob in fact landed as 100644.
2. Agents inventing a second "fix permissions" file outside the requested scope (e.g. a scripts/permissions.sh that runs chmod +x later).

Exposing mode as a first-class field closes both.

create_or_update_file is not changed in this PR because the Contents API has no mode parameter — that's a GitHub API constraint, not a server one. The description now points to push_files for that case.

Test plan

• UPDATE_TOOLSNAPS=true go test ./... — all packages green; toolsnaps for push_files and create_or_update_file regenerated.
• script/lint (golangci-lint v2) — 0 issues.
• script/generate-docs — README.md regenerated to reflect the new files[].mode field description.
• New test cases in Test_PushFiles:
  • successful multi-file push where one entry is 100755 and another defaults to 100644 (asserts the request body sent to the Git Data API contains the correct per-entry mode);
  • rejects an unknown mode string (e.g. 0755) with a clear error;
  • rejects a non-string mode value.

Closes #2578