feat: add clickable file path links in terminal

[AmethystLiang]

Summary

• Add terminal link detection that recognizes file paths (absolute and relative) with optional :line:column suffixes, opening them in the built-in editor or externally
• Split the single shell:openExternal IPC handler into dedicated openUrl, openFilePath, openFileUri, and pathExists handlers with proper URL parsing and path validation
• Register xterm link providers per terminal pane with path existence caching, and support OSC 8 hyperlinks for http(s) and file: protocols
• Add orca:editor-reveal-location custom event so clicking a terminal link jumps the Monaco editor to the exact line/column

Test plan

• Verify clicking a file path like src/main/index.ts:10:5 in the terminal opens the file at line 10, column 5 in the editor
• Verify relative paths (e.g., ./src/foo.ts) resolve against the terminal's cwd
• Verify http/https URLs still open in the default browser
• Verify file: URIs open the file externally
• Verify non-existent paths are not clickable
• Verify paths outside the worktree open externally rather than in the editor

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