feat(agentic): add /goal mode, file-read edit guardrails, and workspace file transfer improvements

[bobleer]

Summary

• Add session goal mode (/goal): synthesize a goal from session context via AI, persist goal state in session metadata, verify achievement after turns, and expose activate_session_goal through desktop API and frontend chat input.
• Enforce file-read-before-edit guardrails: track per-session file read state, reject Edit until the target file was read with a non-partial view and content still matches, and improve agent prompts plus read-line-prefix parsing for reliable edits.
• Improve workspace file handling: robust cross-platform clipboard path/URI parsing (macOS/Linux/Windows), workspace drag-and-drop transfer, and file explorer context-menu / paste flows with tests.

Test plan

• Run /goal in an active chat session and confirm goal text, success criteria, and kickoff message are generated and persisted
• Verify goal continuation and achievement verification across multiple dialog turns
• Confirm Edit tool is blocked until the file is read in-session; succeeds after Read with matching content
• Paste/drag files from clipboard into workspace on macOS (and Linux/Windows if available)
• pnpm run lint:web && pnpm run type-check:web && pnpm --dir src/web-ui run test:run
• cargo check --workspace && cargo test --workspace