feat(memory): integrate optional PowerMem retrieval into system prompt

[Teingi]

Summary

What problem does this PR solve?

• OpenHarness memory is Markdown-first (MEMORY.md + topic files) with keyword/heuristic relevance only. Users who run PowerMem (hybrid vector / full-text / graph retrieval, etc.) had no built-in way to inject PowerMem search results into the assembled system prompt.
• This PR adds an optional retrieval path so agents can use semantic (and hybrid) memory from PowerMem without changing the default behavior for everyone else.

What changed?

• Extended MemorySettings with backend (local | powermem_http | powermem_sdk | hybrid) and PowerMem fields (powermem_base_url, powermem_api_key, powermem_user_id, powermem_agent_id, powermem_run_id), plus env overrides (OPENHARNESS_MEMORY_BACKEND, OPENHARNESS_POWERMEM_*).
• New openharness/memory/powermem_client.py: HTTP search against POST {base}/api/v1/memories/search (via existing httpx); optional in-process SDK via lazy create_memory() when powermem is installed.
• build_runtime_system_prompt (openharness/prompts/context.py): when backend includes PowerMem and there is a latest user message, append a # PowerMem Retrieval section; local / hybrid still use existing Markdown relevance as before.
• pyproject.toml: optional extra powermem → powermem>=1.1.0.
• openharness/memory/init.py: export search_powermem_for_prompt.
• Tests: tests/test_memory/test_powermem_client.py (response parsing + mocked HTTP).

Validation

• uv run ruff check src tests scripts
• uv run pytest -q
• cd frontend/terminal && npx tsc --noEmit (if frontend touched)

Notes

• Related issue: (link the tracking issue, e.g. closes [Feature]: (memory): optional PowerMem retrieval for system prompt context #67 )
• Follow-up work: Document PowerMem in README / README.zh-CN; optional ohmo prompt wiring; MCP or tools for PowerMem write/update if desired; CI job that runs only tests/test_memory/test_powermem_client.py if full suite is heavy (optional).
• SDK mode: Upstream powermem targets Python 3.11+; OpenHarness remains 3.10+ by default — use powermem_http on 3.10, or install the powermem extra on 3.11+ for powermem_sdk.