Rotate GitHub tokens round-robin across posts

[leynos]

Stacked on #151 (→ #149 → #148). Merge order: #148, #149, #151, then this.

What

• New daemon config key github_token_files: an array of file paths, each holding one personal access token. When non-empty, posts rotate round-robin through the pool; the single-token keys still work as a pool of one. Every file is validated (present, non-empty) at startup.
• Every posting-history record now carries token_hash, the hex SHA-256 of the token that made the attempt. Rotation selects the successor (wrapping) of the token whose hash the latest history record carries, falling back to the first token on an empty history or an unrecognized hash — so rotation state survives restarts and token-file renames, and the log never holds a secret.
• comenq hist shows the first eight hash characters per record so you can see which identity posted.

Testing

• Unit: SHA-256 hashing pinned to a known vector; rotation successor/wrap/fallback cases; config pool resolution (ordering, filename naming, inline fallback, empty-file and missing-file validation).
• BDD: two new worker scenarios — posts alternate between two configured tokens (asserting distinct Authorization headers actually reach the mocked GitHub API), and rotation resuming from a seeded history record (wrap from the second token back to the first).
• Full gates green: fmt, clippy + Whitaker dylint, 198 tests + all cucumber features, markdownlint, nixie.

🤖 Generated with Claude Code

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