refactor: map VCS providers by operation

[benvinegar]

Summary

• Replace VCS adapter capability/switch dispatch with explicit per-operation handlers.
• Export concrete adapter implementations (GitVcsAdapter, JjVcsAdapter, SaplingVcsAdapter) through the shared VcsAdapter shape.
• Rename adapter-facing command inputs with a common VCS prefix: VcsDiffCommandInput, VcsShowCommandInput, and VcsStashShowCommandInput.
• Remove the hardcoded VcsMode string union and route default/selected providers through the adapter registry.
• Move untracked-file diff synthesis into adapter-owned paths so loaders.ts no longer branches on provider ids.
• Move Git object/index source reads into src/core/vcs/gitSource.ts, leaving fileSource.ts provider-neutral for shared fetcher primitives and filesystem reads.
• Add shared untracked-file helpers for provider implementations and keep the changeset loader provider-neutral.

Validation

• bun run typecheck
• bun test
• bun run test:integration
• bun run test:tty-smoke
• git diff --check

This PR description was generated by Pi using GPT-5