Add imports inbox heartbeat scaffold

[jshph]

Use case

This is the tenant-side reader scaffold for the Telegram intro backfill pipeline. The control plane can later drop operator-approved Markdown files into imports/**/*.md inside a tenant workspace; this PR gives the agent workspace a safe way to notice those imports during a future heartbeat/manual pass without treating the imported text as instructions.

The concrete first target is Edge Esmeralda intro-channel backfills: each matched tenant can receive a Markdown import summarizing relevant Telegram intro messages, and a future scheduled pass can record that the import exists for later memory/signal workflows.

Summary

• add an imports/**/*.md scanner for future tenant workspace imports
• add a silent ingest prompt that records new/changed import paths and hashes without treating imported Markdown as commands
• document that this is a visible scaffold only: it is not wired to any default cron or heartbeat rollout

Safety / rollout notes

• scanner emits metadata only (path, hash, byte count, status), never imported file content
• state is stored in memory/imports-state.json with restricted file mode
• prompt may only write memory/imports-state.json and memory/YYYY-MM-DD.md; it does not touch USER.md, MEMORY.md, config files, skills, or env files
• no scheduler/heartbeat registration is added in this PR

Validation

• bun test skills/edge-esmeralda/scripts/tests/imports-inbox.test.ts
• bun install --frozen-lockfile
• bun test