docs: add external channel playbook

[saagpatel]

What

• Add a channel-specific external publication playbook for the sanitized AssistSupport demo package.
• Link the playbook from the outbound share package, publication log, and portfolio handoff bundle.
• Add the playbook to the final send and pre-publish checklist path.

Why

• The repo now has a truthful public GitHub package URL recorded, but external channels still need human-operated publishing steps.
• This makes the remaining portfolio, LinkedIn, email, DM, and video lanes executable without storing private recipient or account data in repo docs.

How

• Documented channel decision criteria, runbooks, safe log-entry templates, and pre-send verification.
• Kept all examples tied to fictional Northstar Labs demo data and generic sent-channel notes.

Testing

• Verified relative markdown links for the changed demo docs.
• Ran sanitized-boundary scans for stale company/demo terms; both returned no matches.
• Ran node scripts/ci/check-workstation-preflight.mjs, node scripts/ci/check-workflow-command-drift.mjs, and node scripts/ci/check-version-parity.mjs.
• Ran pnpm health:repo:checks && pnpm ui:gate:static.
• Ran pnpm git:guard:all.

Risks

• Low. Documentation-only change; no external publication is claimed or performed.

Performance

• No runtime or bundle impact.

Lockfile

• No lockfile changes.

Screenshots

• Not applicable; no UI changes.