Fire a first-contact event after initial agent attach

[lilyshen0722]

Closes #686.

Summary

• add an append-only AgentFirstContact marker with a unique (userId, agentName, instanceId) index, independent of replaceable AgentInstallation rows
• atomically enqueue first_contact only for the first human-to-agent-identity relationship install; reinstall, concurrent install, second-pod install, and agent-to-agent install do not replay it
• treat duplicate-key losers in a concurrent upsert race as the expected no-op instead of logging a false install error
• deliver the exact inline first-impression cue through AgentEvent, with install remaining best-effort if marker/event work fails
• teach both the native runtime and local CLI wrapper to consume first_contact as a model prompt; the OpenClaw and webhook poll paths are already type-generic
• make existing integration tests select the event type they exercise now that a fresh install also queues first_contact

Identity semantics

• First contact is once per durable agent identity: human userId + agentName + instanceId. Aria and Pixel each greet once even though both use the openclaw runtime.
• Installing the same identity into another pod does not replay the greeting; uninstall/reinstall also remains silent because the marker outlives AgentInstallation.
• The existing deterministic team-pod install intro is unchanged. A first attach in a team pod can therefore produce that system-authored intro plus the agent-authored first-contact greeting. I left this visible rather than silently changing the older flow in this PR.

Verification

• focused correction set: 4/4 suites, 13/13 tests passed
• backend full suite on the original head: 172/172 suites, 1,235/1,235 tests passed (Jest printed its known open-handle warning after the green summary; the idle container was then stopped)
• affected integration suite: 23/23 passed
• CLI full suite: 15/15 suites, 156 passed, 8 skipped
• backend TypeScript check and production build passed after the identity-key correction
• real Mongo probes after the correction:
  • 8 simultaneous calls for the same identity produced 1 marker and 1 event
  • Aria + Pixel for the same human produced 2 markers and 2 events
• git diff --check passed
• new JS test files pass ESLint when the repository's broken TS import-resolution rules are disabled

Known repository baseline: backend npm run lint currently reports 1,318 existing parser/import-resolution/style problems across the tree, and the CLI lint script has no eslint dependency/config. This PR does not attempt that unrelated lint migration.