feat(changelog): BitRouter-refined drafts + multi-channel announce

[SPIKESPIGEL404]

What

Extends the existing changelog sync into the full refine → gate → publish loop for both BitRouter (OSS) and BitRouter Cloud, per the two-repo design (product repos just dispatch; all refinement/marketing lives here).

release:published (bitrouter | bitrouter-cloud)
  → repository_dispatch: bitrouter-release  (product repo — see companion PR)
  → sync-changelog.mjs + BitRouter refine ──► draft content/changelog/<ver>.mdx
  → PR "changelog/sync"  (unchanged review gate — curate + merge)
  → site rebuild → changelog page + RSS/Atom     (docs publish, already exists)
  → announce-channels.yml → Discord · X (xurl) · Resend   (NEW)

Changes

File	Change
scripts/sync-changelog.mjs	Optional BitRouter refine pass (dogfood) → polished prose + title/description. Deterministic fallback with no key. New cloud product path (inline body, cloud-<slug>.mdx).
source.config.ts	changelog frontmatter gains optional product: oss | cloud.
.github/workflows/sync-changelog.yml	Pass product / inline body / BITROUTER_API_KEY from the dispatch.
scripts/announce-channels.mjs (new)	Fan out a merged entry to Discord / X / Resend; per-channel copy optionally BitRouter-spun; graceful skips; version tiering; backfill age-guard.
.github/workflows/announce-channels.yml (new)	Runs the fan-out on merged content/changelog/**; once-only via announced/<slug> tags.

Tiering: *-alpha/beta/rc* & patches → Discord only; x.y.0 → Discord + X + email.

Safety

• Non-breaking: no BITROUTER_API_KEY → the existing deterministic draft; no channel secret → that channel is skipped (logged, non-fatal). Safe to merge before anything is wired.
• Idempotent: each entry announced once (announced/<slug> tag); backfill merges older than 14d are skipped.
• Review gate (changelog/sync PR) and docs/RSS publish are unchanged.

Companion change (product repos)

Needs bitrouter/bitrouter (and bitrouter-cloud) to fire the release dispatch so sync runs on release instead of the daily cron — a small notify-release.yml (already drafted on the bitrouter side). Cloud sends product: cloud + inline body since this repo can't read the private repo.

Secrets / vars to add here

Secrets: BITROUTER_API_KEY (refine + channel copy), DISCORD_WEBHOOK_OSS, DISCORD_WEBHOOK_CLOUD, XURL_TOKENS, RESEND_API_KEY
Variables: SITE_URL, RESEND_AUDIENCE_OSS, RESEND_AUDIENCE_CLOUD, RESEND_FROM

Tested

• node --check + YAML parse on all files.
• Deterministic sync of v1.0.0-alpha.22 → valid .mdx with product frontmatter.
• Fan-out dry-run: prerelease → Discord only; x.y.0 cloud → all three; every channel skipped gracefully with no secrets; age-guard drops a 556d-old entry. All exit 0.

Verify before enabling (couldn't test without live creds)

• The refine LLM call path (fetch + JSON parse) — proven by the deterministic fallback + parser, but not exercised against a live model here.
• xurl install URL / token-store path in announce-channels.yml — pin to your version.
• Resend Broadcasts shape + a verified RESEND_FROM domain.

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