fix(agent): enable virtual_mode for default filesystem backend

[rlancemartin]

Problem

When the agent creates files with paths like /research_plan.md, it tries to write to the root filesystem instead of the current working directory, resulting in errors:

Error writing file '/research_plan.md': [Errno 30] Read-only file system: '/research_plan.md'

Root Cause

The default FilesystemBackend in agent.py was created without virtual_mode=True:

backend = CompositeBackend(
    default=FilesystemBackend(),  # No virtual_mode
    routes={"/memories/": long_term_backend}
)

Without virtual_mode=True, the FilesystemBackend treats paths starting with / as absolute system paths rather than paths relative to the working directory.

Solution

Enable virtual_mode=True for the default backend:

backend = CompositeBackend(
    default=FilesystemBackend(root_dir=Path.cwd(), virtual_mode=True),  # With virtual_mode
    routes={"/memories/": long_term_backend}
)

With this fix:

• /research_plan.md → {cwd}/research_plan.md (sandboxed to working directory)
• /memories/file.md → Still routes to agent directory via /memories/ route

Impact

• Safer: File operations are sandboxed to the current working directory
• More intuitive: Paths like /file.txt behave as users expect (relative to cwd)
• No breaking changes: /memories/ routing continues to work as before

Testing

Verified with test script:

backend = FilesystemBackend(root_dir=Path.cwd(), virtual_mode=True)
path = backend._resolve_path("/research_plan.md")
# Result: /tmp/research_plan.md (writable) 

Without fix:

backend = FilesystemBackend()
path = backend._resolve_path("/research_plan.md") 
# Result: /research_plan.md (system root, not writable)