refactor: abstract-filesystem-interface

[Salmatcre8]

Summary

Introduces a unified FileSystemProvider interface and three concrete adapters so the IDE's file operations compile against an abstraction rather than reaching for browser storage APIs directly. All changes are confined to the ide/ directory.

Closes #882

Deliverables

• ide/src/lib/fs/FileSystemProvider.ts — the unified interface (readFile, writeFile, delete, exists, list, stat) plus a FileStat type, a FileNotFoundError, and path helpers (normalizePath / dirname / basename) with ..-traversal protection.
• Adapters (ide/src/lib/fs/adapters/):
  • BrowserFileSystemProvider — IndexedDB-backed, for in-browser workspaces (replaces ad-hoc localStorage/IndexedDB access). Directories are implicit from stored paths.
  • LocalFileSystemProvider — Node fs rooted at a directory, with root-containment checks; fs impl is injectable for testing.
  • GitHubFileSystemProvider — remote GitHub repo via the REST Contents API (base64 blob decode on read; create-or-update with blob sha on write); fetch is injectable.
• ide/src/lib/fs/index.ts — barrel export for clean imports.

Each adapter implements the identical FileSystemProvider contract, so workspace/editor code can be written once and pointed at browser, local, or remote storage.

Design notes

• Paths are normalised to POSIX form internally; a leading slash is optional and equivalent to none.
• External dependencies (node fs, fetch) are injected via constructors, keeping the adapters unit-testable without real I/O.
• Aligns with the existing ide/src/lib/fs/UniversalResolver.ts conventions (same folder, IndexedDB usage style, vitest tests under __tests__/).

Verified terminal output

This is a TypeScript library change (no UI to screenshot), so here is the test + typecheck output:

# Unit tests (vitest) — interface helpers + all three adapters
 Test Files  1 passed (1)
      Tests  25 passed (25)

# Typecheck — no errors introduced in src/lib/fs
$ npx tsc --noEmit  ->  0 errors in src/lib/fs/**
# (one pre-existing, unrelated error in src/lib/test/FuzzingEngine.ts exists on main)

The 25 tests cover: path normalization + traversal guards; Browser adapter read/write/delete/exists/list/stat with implicit directories; Local adapter via an injected in-memory fs (incl. root-escape rejection); GitHub adapter via injected fetch (base64 read, 404 → FileNotFoundError, create-without-sha, update-with-sha, list, exists); and a cross-adapter contract test asserting all three implement the full interface.

Acceptance criteria

• Unified FileSystemProvider interface
• Adapters for browser storage, local files, and remote repositories
• Deliverable src/lib/fs/FileSystemProvider.ts present
• All work confined to ide/

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