idle-draft: fill all free worker slots, not one per completion

try_submit() returned after a single submission and was only called at
startup and once per completion, capping real concurrency at 1 task
regardless of the parallel setting — the ThreadPoolExecutor pool was
sized but never filled. Loop until every free slot is filled or no
eligible candidate remains, per spec §10 (wording sharpened to make the
whole-pool semantics explicit). No stub-claude harness exists yet to
test dispatch concurrency end-to-end; verified live against the real
queue (36 candidates, parallel=20).

Claude-Session: https://claude.ai/code/session_01Lgv4Qn82boNFC1jn8QXSNw

specs/idle-draft.spec.md

@@ -218,8 +218,10 @@ next cycle); the walk continues to the next candidate.
 Up to `parallel` tasks run concurrently (`concurrent.futures.ThreadPoolExecutor`).
 Each completion is handled **serially** in the main thread (state updates and git
 commits never race). On every completion — and before the very first dispatch — the
-gates (§6) are recomputed from a fresh probe (§5); a task is submitted to fill a free
-worker slot only while an eligible `(item, work_type, provider)` triple remains. The
+gates (§6) are recomputed from a fresh probe (§5); tasks are then submitted until
+**every** free worker slot is filled or no eligible `(item, work_type, provider)`
+triple remains — one completion may therefore trigger multiple submissions, and the
+initial call before the first dispatch fills the whole pool, not one slot. The
 loop exits (dispatch mode, non-dryrun) when no eligible candidate remains. `--once`
 dispatches at most one task total, then exits without waiting for further slots.