feat(memory): auto-migrate shared root memories to per-user namespace dirs

[Laurc2004]

Summary

When per-user memory isolation is enabled, users with existing shared memories (MEMORY.md / USER.md in ~/.hermes/memories/ root) would lose their memories because the new namespaced directory starts empty.

This PR adds automatic migration: on first load for a new namespaced user, shared root memory files are automatically copied into their per-user directory.

Changes

tools/memory_tool.py

• Add _migrate_from_shared() — helper that copies MEMORY.md and USER.md from the shared root to a namespaced user directory, only if the target files do not already exist (no overwrite)
• Fix load_from_disk() in MemoryStore.__init__ area — the original definition used get_memory_dir() without the namespace parameter, which would break per-user isolation. Fixed to use get_memory_dir(self.namespace) and call migration.
• Remove duplicate load_from_disk() — the second definition shadowed the first, making the fix in the first ineffective
• Fix regex escaping in get_memory_dir() (@.+ → @\\.+ for proper literal dot matching)

tests/tools/test_memory_tool.py

• TestMemoryMigrationFromSharedRoot — new test class with 3 tests:
  • test_migration_copies_shared_files_to_new_namespace — verifies happy path
  • test_migration_does_not_overwrite_existing_user_files — verifies no data loss
  • test_migration_does_nothing_for_empty_namespace — verifies CLI/global sessions unaffected

Testing

All 37 tests in test_memory_tool.py pass, including:

• 3 new migration tests
• Existing namespace isolation test
• All existing MemoryStore and memory_tool dispatcher tests

[Laurc2004]

相关issue引用：

• Feature: Per-user memory isolation in group chats to prevent identity contamination #11430 — 群聊memory跨用户身份污染
• session_search should be scoped to the current chat by default in gateway conversations #10554 — session_search 应按当前聊天范围隔离
• Feature: Single-Daemon Multi-Agent with Per-Topic Workspace & Memory Isolation #9514 — 单守护进程多Agent：按Topic隔离工作区和记忆
• Feature: Per-topic/per-group system prompt (or SOUL.md) binding for Telegram #5195 — Telegram 按Topic/群绑定自定义system prompt
• [Bug]: background process watch notifications can leak into the wrong threaded session #9532 — 后台进程通知泄漏到错误的session（已关闭）

此PR通过per-user namespace实现了内存隔离，解决了群聊中用户身份混淆的问题，并为后续的session搜索隔离和多Agent架构提供了基础。

[teknium1]

Thanks for tackling a real multi-user memory boundary.

Problems

• Current main still has the built-in-memory leak, but its construction moved: agent/agent_init.py:1341-1345 creates MemoryStore without gateway identity. The PR's run_agent.py initialization hunk is stale, and GitHub reports this branch as conflicting.
• Copying shared MEMORY.md / USER.md into every first-seen platform:user_id namespace cannot safely migrate ownership: the shared files have no user provenance. It can replicate one user's historical profile to unrelated gateway users.
• The gateway approval fallback remains global: gateway/slash_commands.py:2765-2789 calls load_on_disk_store() without source identity, while tools/memory_tool.py:791-820 accepts no scope. Approved staged writes would bypass the proposed namespace.
• The PR also includes unrelated streaming-delivery changes. Current final-delivery reconciliation is now implemented at gateway/run.py:19323-19341 and gateway/run.py:19904-19952; review and salvage that work separately.

Suggested changes

• Rehome scope construction in agent/agent_init.py and thread one validated scope through all direct-store and approval paths.
• Use an explicit, owner-qualified migration policy rather than copying an unscoped USER.md to every user.
• Add temp-HERMES_HOME integration tests for two gateway users and staged-write approval.

This is an automated hermes-sweeper review; maintainers will make the final call.