refactor(lana): use URI-safe file access

[peternhale]

PR overview

Stack 2 of 4. Depends on #951. Makes log and workspace handling URI-native so Lana can operate against local and virtual filesystems.

Changes made

• Replace filesystem paths with VS Code Uri values or serialized URI strings.
• Route reads, writes, existence checks, caching, and navigation through URI-safe services.
• Support file, memfs, vscode-vfs, and other virtual workspace schemes.
• Update log analysis, language detection, providers, source lookup, and workspace selection.
• Update URI mocks and affected unit tests.

Type of change

• Refactor

Related issues

related W-23939830

Validation

• pnpm typecheck
• Seven focused unit suites: 98 tests