feat(agentcfg): agent instance store (8/15)

[ccf-lisa]

Part 8/15 of the agent remote-configuration stack

This stack splits #465 into reviewable layers of at most ~1000 changed lines each (counted without docs/, go.sum and Markdown). The last layer's tree is identical to #465, which already has its review history.

Stacked on #475 (lisa/agent-config/07-revision-store). Review and merge in order.

What's in this layer

Eighth layer of the agent remote-configuration stack (split from #465): the ccf_agent_instances model and the agentcfg instance API: UpsertReport (config reports) and TouchFromHeartbeat, the per-agent instance cap (prune-eligible rows don't count; the oldest stale instance is replaced; a short-lived cap-reached cache for heartbeats), ListInstances/GetInstance, and the derived status/sync-status/staleness rules.

Size: +982 -4 = 986 changed lines (without docs/go.sum).

Verification

Each layer builds on its own: go build, go vet (also with -tags integration), golangci-lint run and go test ./... pass, and make swag leaves the tree clean. Integration suites for the packages this layer touches pass locally on testcontainers Postgres.

🤖 Generated with Claude Code

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