Render propose's --from literally in the preamble and drop the CLI's VORO_TASK_ID read

[MJohnson459]

Render propose's --from literally in the dispatch preamble and remove the CLI's VORO_TASK_ID env read, so voro's own logic consults no environment.

Problem

A dispatched session runs with VORO_TASK_ID exported. The binary read it in one place — the propose --from default — so cargo test inside a dispatched session leaked the ambient id into the CLI tests: a scratch-db propose picked up VORO_TASK_ID as its discovered-from source and errored "task N not found". CI passed only because the var is unset there. It had bitten twice.

Change

• dispatch.rs: the return-path preamble now renders the propose line with a literal --from {task_id} (matching ask/done), and the prose describes the rendered flag instead of an env fallback. Dropped DispatchCtx::session_task_id.
• cli.rs: propose_verb takes no ambient source — discovered-from comes only from an explicit --from. run() reads no environment.
• main.rs: removed the VORO_TASK_ID read.
• Dispatch still exports VORO_TASK_ID (dispatch.rs:499) solely so the Claude Code shell hooks can expand it into explicit args; nothing in voro's logic reads it, making the test leak impossible by construction.
• docs: DESIGN.md §8 and agent-integration.md describe the rendered --from; the operator-pasted snippet now passes --from "$VORO_TASK_ID" explicitly so it still links back.

Verification

• cargo test --workspace, cargo clippy --workspace --all-targets -- -D warnings, cargo fmt --all --check — all green.
• VORO_TASK_ID=999 cargo test --workspace — now passes (previously failed); confirmed the formerly-failing project_archive_hides_the_cockpit_views_and_unarchive_restores_them passes under it.
• New integration test tests/propose_ignores_ambient_task_id.rs runs the built binary under VORO_TASK_ID=999: a bare propose links nothing, --from 1 still links. Preamble unit test asserts the literal --from {id} and no VORO_TASK_ID in the prompt.

Branch: worktree-cli-drop-voro-task-id-env. Not pushed (bg session).