Slack: app manifest and authenticated bounded ingress envelope

[jeffhuber]

Part of #903.

Problem

Code Mower needs a minimal Slack app surface and an authentication boundary that can acknowledge quickly without dispatching work inline.

Scope

Add a manifest template plus raw-body HMAC verification, five-minute replay rejection, URL verification, strict body/field bounds, three-second receipt acknowledgement, and offline fixtures. Keep the implementation transport-neutral and route accepted envelopes to a durable interface.

Dependencies

• Slack: freeze command, identity, lifecycle, privacy, and event contracts #916

Acceptance criteria

• Invalid signatures, stale timestamps, duplicate requests, oversized bodies, unknown commands, and malformed payloads are rejected safely.
• A valid receipt is persisted before acknowledgement.
• Minimum scopes exclude history, files, email, user tokens, and public posting.
• No task is dispatched by the HTTP request handler.

Code Mower delivery

Produce exactly one independently reviewable PR for this issue. Record the named builder, keep one writer on the branch, run focused tests and applicable full checks, obtain an independent Code Mower review against the exact current head, resolve every P0/P1/P2 finding, and pass normal CI plus code-mower/gate. Update the parent epic with PR/head, review, validation, outcome, and safe metadata-only upload evidence. Do not put credentials, source, diffs, prompts, transcripts, private context, task/message prose, or raw provider output in cloud data.