Name a quick-propose session for its project, not its id

[MJohnson459]

A quick propose (n) launched its headless agent under voro-propose-<project_id>, while the interactive planning session beside it (N), on the same project, was named voro-plan-<project>. In claude agents the two read as different kinds of thing, and the bare number was the one shape a Voro-composed session name otherwise reserves for a task id.

Launch::Propose now carries the project's name, sanitised through the same sanitize_for_name the Plan arms use, so a quick propose on the voro project is voro-propose-voro with slug propose-voro, and odm 2 reduces to voro-propose-odm-2 exactly as a planning session's does. dispatch::propose() builds the launch from the project it already loads; the project_id parameter stays, since it is what the caller has and what default_repo needs. The variant's doc comment, which used to argue that the id served because propose already occupied the position a bare number would be read in, now says what Plan's says: a task-less launch names its project by name, and the two share one convention.

Nothing else about the quick propose moves — it still runs the dispatch verb, opens no session row, moves no state, and still refuses an agent whose dispatch template carries {task_id}.

Docs updated in the same change, per CLAUDE.md: DESIGN.md §8's {session_name} paragraph now states the invariant over both task-less launches rather than planning alone; docs/agent-integration.md's {session_name} bullet lists the propose form beside the others, keeping the "bare number is always a task id" justification that now covers it; the embedded voro.toml starter comment gained the form; CHANGELOG.md has a ### Changed entry.

Tests: a_quick_propose_names_its_project_and_has_no_task rewritten against a project name (voro-propose-mote / propose-mote, still no task id, still distinct from a dispatch's name); the plan sanitising test extended to cover both variants and renamed a_task_less_launch_sanitizes_its_project_name; and a new dispatch::tests::propose_names_its_session_for_the_project proves end-to-end, through a stub agent that echoes {session_name} to a marker, that a project named odm 2 spawns voro-propose-odm-2 and stamps its prompt file propose-odm-2-*.

Verified: cargo test --workspace (859 tests, 0 failed), cargo clippy --workspace --all-targets -- -D warnings clean, cargo fmt --all --check clean, and rg 'voro-propose|propose-\{?project_id' shows no stale text.