[AGENT-HOMES-001 PR-4] Per-machine capability grants (default deny)

[mfethe1]

Stacks on #15 (PR-3). Base is the PR-3 branch — retarget to product/main after #15 merges.

What

The authorization half of agent homes: which agent may perform which capability against which target machine.

• migrations/0048_agent_capability_grants.sql — tenant-scoped, community-fenced grant table
• store/agent_capability_grants.rs — grant / revoke / is_granted / list_grants + impl Db methods
• api/mod.rs::require_capability — relay-side gate returning 403 capability_not_granted
• 2 new audit actions: capability_granted / capability_revoked

Design decisions

• Default deny. Absence of a grant is a denial. is_granted returns false unless an active, non-revoked row exists.
• Not relaxed by require_relay_membership. An open relay still denies ungranted cross-machine actions — unlike enforce_relay_membership, which no-ops on open relays. Cross-machine execution is a strictly higher bar than reading a relay.
• Revocation is a tombstone, not a delete: revoked grants stay visible to list_grants and the audit trail. Re-granting reactivates the row.
• Community-scoped throughout. A grant in one community never authorizes anything in another (test enforces this).

Verification (real Postgres, not mocks)

• 8/8 store tests: default-deny, revoke round-trip, re-grant clears tombstone, wildcard target, no leak across capabilities, no leak across targets, community isolation, revoked-still-listed
• 13/13 migration lint tests pass — including all_non_operator_global_tables_have_not_null_community_id and scoped_primary_key_..._lead_with_community_id, which validate the new table
• buzz-audit round-trip test covers both new variants
• fmt + clippy clean on buzz-db, buzz-audit, buzz-relay

Reviewer note

Sensitive-path (authz) → Path B, review required per AGENTS.md.

require_capability has no production call site yet — no cross-machine endpoint exists. I deliberately did not invent one; the gate lands with the model so the endpoint PR can consume it.

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