Add path-scoped change explanations

[Naseem77]

Summary

• add repeatable --path PATH support to shiftory explain and shiftory analyze
• select files exactly and directories recursively, unioning multiple selections across working-tree and committed comparisons
• reject outside-repository, traversal, repository-root, and unmatched selections with typed diagnostics
• bind normalized selections to comparison identities and resumable explain descriptors
• keep Graphora indexing on complete before/after repository snapshots while limiting changed-path and changed-line enrichment focus to selected changes
• update the bundled/source Shiftory skill and documentation while preserving unscoped behavior

Validation

• Ruff formatting and lint checks on changed Python files
• strict mypy across all 29 source files
• 90 targeted tests passed, 1 environment-dependent Graphora test skipped
• repository schema, link, skill, and license-material validation
• focused committed-scope CLI smoke selecting one file and one recursive directory

Compatibility

Omitting --path preserves existing comparison identities and full-comparison behavior. Source and packaged skill copies remain byte-identical.