fix(server): give create-content's commitFile a stable git identity

[davidwkeith]

Summary

• create_page/create_post/createTyped (via commitFile in server/create-content.mjs) shelled out to plain git commit with no GIT_AUTHOR_*/GIT_COMMITTER_* env configured.
• git commit's auto-detect identity fallback needs a passwd/GECOS entry to synthesize a name/email; the Apple Containerization guest used by Anglesite/Anglesite-app has none. Every commit there silently failed, and commitFile's catch-all swallowed the failure into commit: null — the file was written correctly, but the commit was dropped with no error surfaced to the MCP caller.
• server/edit-history.mjs and server/undo-edit.mjs already solved this for their commit-tree calls by passing ANGLESITE_COMMIT_IDENTITY (server/git-identity.mjs, recordEdit's commit-tree has no git identity, so every container overlay edit silently fails to commit #428). This applies the same fix to commitFile.
• Adds a commitFile — no ambient git identity (#428) test block to test/create-content.test.js, mirroring the equivalent regression test in test/edit-history.test.js, covering createPage, createPost, and createTyped.

Paired PR check

• This change is self-contained to the plugin (skills / hooks / template / docs / MCP server with no consumer-visible contract change). commit was already a nullable field in every response; this only makes it non-null in more cases (inside the container guest) than before. No paired app PR needed.
• This change needs a paired PR in Anglesite/Anglesite-app (MCP message schema, template fields the app reads, hook surface, anything the native app embeds or shells out to).

Test plan

• npm test — full suite passes (156 files / 3141 tests / 1 todo)
• New tests fail-safe under the exact #428 container conditions (no user.name/user.email config, no GIT_AUTHOR_*/GIT_COMMITTER_* env, GIT_CONFIG_NOSYSTEM=1), reproducing the sibling test pattern from test/edit-history.test.js