fix: support short-format URIs in VikingURI and VikingFS access control

[r266-tech]

Description

Fix VikingURI and VikingFS._is_accessible() to accept short-format URI paths (e.g., /resources, user/memories) in addition to full-format URIs (viking://resources).

Related Issue

Fixes #259

Root Cause

Two issues prevented short-format URIs from working:

1. VikingURI.__init__ required URIs to start with viking://, raising ValueError for short paths like /resources
2. VikingFS._is_accessible() returned False (access denied) for any URI not starting with viking://, which triggered PermissionError before _uri_to_path could normalize the path

Changes Made

File	Change
openviking_cli/utils/uri.py	VikingURI.__init__ now auto-normalizes via the existing normalize() static method
openviking/storage/viking_fs.py	_is_accessible() normalizes non-viking:// URIs instead of rejecting them
tests/unit/test_uri_short_format.py	19 tests covering short-format normalization, scope validation, join/parent operations

Design Decisions

• Leverages existing normalize() method: The VikingURI.normalize() static method already handled the conversion correctly — the fix simply wires it into the constructor
• Backward compatible: Full-format URIs pass through normalize() unchanged (idempotent)
• Invalid scopes still rejected: VikingURI("/invalid_scope/foo") still raises ValueError — only the format requirement is relaxed

Testing

# Before: ValueError / PermissionError
VikingURI("/resources")           # ValueError
client.ls("/resources")           # PermissionError: Access denied

# After: works correctly
VikingURI("/resources").scope     # "resources"
VikingURI("/resources").uri       # "viking://resources"

All 19 new tests pass. No existing tests affected.

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Checklist

• My code follows the project coding style
• I have performed a self-review of my code
• New and existing unit tests pass locally with my changes
• My changes generate no new warnings

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