feat(agent-content): add stable memory carrier API

[yeyitech]

Summary

• add a dedicated /api/v1/agent-content API for stable named agent memory carriers
• support create, replace, append, and merge flows while preserving generic /api/v1/content/write semantics
• expose the same capability through embedded SDK entry points and add focused server/service tests

Details

• restrict the new API to agent-scoped markdown files under viking://agent/<agent_space>/memories/<category>/...
• allow create-if-missing and error-if-exists carrier registration semantics
• reuse memory refresh plumbing after writes and reuse MemoryExtractor._merge_memory_bundle() for merge, with append-style fallback when structured merge is unavailable

Verification

• python -m compileall openviking/async_client.py openviking/client/local.py openviking/server/routers/agent_content.py openviking/service/fs_service.py openviking/storage/content_write.py openviking/sync_client.py tests/server/test_api_agent_content.py tests/server/test_agent_content_service.py tests/server/test_content_write_service.py
• manual embedded/ASGI verification for service + endpoint + SDK flows (create/append/merge)
• note: targeted pytest is currently blocked in this repo by the existing pytest-asyncio collection bug: AttributeError: 'Package' object has no attribute 'obj'

Fixes #1251

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PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

🎫 Ticket compliance analysis 🔶 1251 - Partially compliant Compliant requirements: Add a dedicated /api/v1/agent-content API for stable named agent memory carriers Support create, replace, append, and merge flows Preserve generic /api/v1/content/write semantics Expose capability through embedded SDK entry points Add focused server/service tests Restrict API to agent-scoped markdown files under viking://agent/<agent_space>/memories/<category>/... Allow create-if-missing and error-if-exists carrier registration semantics Reuse memory refresh plumbing and MemoryExtractor._merge_memory_bundle() for merge Non-compliant requirements: (no items) Requires further human verification: (no items)

⏱️ Estimated effort to review: 4 🔵🔵🔵🔵⚪

🏅 Score: 90

🧪 PR contains tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ No major issues detected

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