fix(session): skip vector seed for session directories

[qin-ctx]

Description

Prevent preset user session directories from seeding L0/L1 vector records during user directory initialization. Session directories are still created in VikingFS for discovery and migration, but they no longer enter the semantic vector index at creation time.

Also keep the short viking://user URI as the generic user namespace container instead of silently resolving it to the current user. Write operations now reject viking://user and require an explicit user namespace or current-user content path.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• Skip preset L0/L1 vector seeding for URIs classified as session namespace paths.
• Keep existing session directory creation behavior unchanged.
• Reuse the existing is_session_uri namespace rule so initialization matches reindex/indexing exclusions.
• Preserve viking://user as a short user namespace container URI during canonicalization.
• Reject writes to viking://user with a clear permission error instead of resolving it through the current identity.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Validation run:

• .venv/bin/ruff format openviking/core/directories.py --check
• .venv/bin/ruff check openviking/core/directories.py

Not run per request:

• Unit tests

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A

Additional Notes

Tests were not added or run per request.

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