Extract a shared gh_put_file() helper for contents-PUT-with-sha

[drmowinckels]

Context

Several modules independently implement the same "create or update a file on a branch via the GitHub Contents API, handling the existing sha" pattern:

• R/blog-add.R — PUT /repos/{owner}/{repo}/contents/{path} (no sha check)
• R/contributors.R — GET existing, compare content, PUT with sha
• R/chapter-onboard.R — base64-encode + PUT
• R/airtable-sync.R — PUT with sha
• R/copilot-sync.R — copilot_put_file() (GET, skip-if-unchanged, PUT with sha)

Flagged by the reuse pass of a /simplify review on #80. copilot_put_file() is effectively the 4th–5th copy. The variants differ subtly (some skip the GET, some compare content before encoding), so bug fixes and behaviour (e.g. no-op skipping) don't propagate.

Proposal

Extract one gh_put_file(owner, repo, path, content, branch, message) into R/gh.R that:

• GETs the existing file on branch (tolerating 404),
• returns early (no commit) when the decoded existing content is identical,
• base64-encodes and PUTs, passing sha when the file exists,
• returns whether a commit was pushed.

Then migrate the call sites to it. Each migration must preserve that module's current behaviour (e.g. blog-add's create-only semantics) and keep its tests green.

Notes

• Out of scope for Summon GitHub Copilot to run grimoire review gates #80 (touches unrelated modules); tracked here to avoid a risky cross-module refactor inside a feature PR.
• Reuse base64_decode() (R/i18n.R) for the unchanged-check.