feat(opencode): add turn-scoped memory recall

[LeXwDeX]

Summary

• keep first-message Topic recall optional and scope injected memory to the current real user turn
• add root-main-agent-only memory_search with acknowledgement-only durable output
• isolate child/disabled/ineligible sessions and discard stale concurrent results
• decouple periodic maintenance cadence from retrieval-context lifetime

Verification

• focused MEMORY/tool/registry tests: 44 pass, 0 fail
• prompt tests: 74 pass, 1 skip, 0 fail
• compaction tests: 54 pass, 1 skip, 0 fail
• packages/opencode: bun typecheck
• repository lint: 4851 warnings, 0 errors (ratchet 4852)
• Prettier, diff-check, and strict OpenSpec validation
• Standards review: no findings
• Spec review: no findings

No HTTP API, generated SDK, or persisted Topic schema changes.