feat(examples): add provider-agnostic Last30Days agent

[bossriceshark]

Summary

Adds an examples-only reference agent that implements /last30days <topic> through the existing ACP slash pass-through from #919 — no core, relay, or Desktop changes.

The example runs 10 independent analysis workers plus one synthesis call against any OpenAI-compatible endpoint, then publishes one sourced brief back to the originating thread. It follows the focused-reference style of examples/countdown-bot (#516) and the persona-pack layout of examples/meadow-core.

Closes the proposal in #4158.

Why this shape

• Command semantics stay in the connector. feat(acp): pass slash commands through to ACP connectors #919 deliberately keeps meaning out of core; this exercises that boundary rather than extending it.
• No overlap with discovery/autocomplete work in feat(desktop): slash-command autocomplete in composer from ACP available_commands_update (follow-up to #919) #2528 / feat(desktop): autocomplete ACP slash commands (rebase of #2653) #3537.

Provider-agnostic by design

Setting	Default	Notes
OPENAI_API_KEY	(required)	Adopter-supplied. Read from the process environment only — no .env discovery, no bundled credentials.
OPENAI_BASE_URL	https://openrouter.ai/api/v1	Any OpenAI-compatible base. Changing this sends your key to that host — documented as an explicit trust decision.
model	deepseek/deepseek-v4-pro	Documented working default; any model slug works.
workers	10	Under --enforce-gates, min-success always equals the worker count.

Safety properties

• No shell execution. The optional evidence hook takes a JSON argv array, runs with shell=False, and treats the topic as one opaque argv element. Shell-string templates are rejected, as are shell-interpreter -c templates containing placeholders.
• Opaque topic I/O via --topic-stdin / --topic-file; no documented invocation interpolates a topic into a shell line.
• Abuse and cost gates on by default under --enforce-gates: event-id idempotency, per-requester cooldown and daily quota, global daily spend ceiling with per-run reservation, and a concurrency cap. Gate state is a single file written with temp + fsync + atomic os.replace under a lock, and fails closed on unparseable state.
• Secret hygiene: metadata-only receipts (no topic, brief text, absolute paths, or requester identity), sanitized public errors with secret and filesystem-path redaction, and private artifacts (0700/0600).

Testing

PYTHONDONTWRITEBYTECODE=1 python3 -B examples/last30days-agent/scripts/test_last30days.py
# 68 passed

Offline and fully mocked — no network, no provider calls, no credentials required. Coverage includes injection regressions (malicious topics stay inert; shell templates rejected), gate transactionality (torn state fails closed, replace-failure preserves prior state), min-success enforcement, and redaction.

Manual test steps are in examples/last30days-agent/README.md (quickstart, config table, security notes).

Known follow-ups

• Evidence gathering is intentionally pluggable; no gatherer ships by default, so the example runs topic-only unless one is configured.
• Gate state is single-node (file-locked); a multi-host deployment would want shared storage.