fix(ha): claim workflows on spawn + persist running to stop the "untriggered" steal (#89)

[gustavobertoi]

Closes the root cause of #89 (the e2e quarantine was only a workaround). One PR to main.

Root cause

On the 3-node HA stack, a freshly triggered workflow intermittently stayed untriggered forever:

• the trigger path never set claimed_by and never persisted the untriggered → running
  transition (only journaled it), so the row sat at claimed_by=NULL, state=untriggered for the
  whole run;
• a Postgres NOTIFY on the untriggered INSERT makes every node's claim actor sweep immediately,
  and the sweep claims claimed_by IS NULL AND state IN ('untriggered','running','sleeping') → another
  node stole and re-triggered it → a duplicate WorkflowHandler → stall;
• recoverWorkflows (running/sleeping only) never healed it, and GET /v1/workflows/{id} reported
  the running workflow as untriggered.

Fix

• ClaimRepository.ClaimWorkflow(nodeID, workflowID) — atomically claim a single workflow for
  the owning node (memory no-op → true; postgres UPDATE … RETURNING rows-affected).
• WorkflowHandler.Init claims the workflow for this node on both the new-trigger and replay
  paths; on loss to another live node it bows out (signals the instance supervisor it's done
  here) so no duplicate runs. It then persists the running/sleeping state immediately.
• The sweep's lease-expired branch and recoverWorkflows now include untriggered so a
  crash-stranded row is reclaimed/re-driven.
• ADR-0018 amended with the ownership invariants.

Safety

Non-HA / memory: no behavior change (claim is a no-op; the claim actor + PgListener are HA-gated).
The e2e re-trigger quarantine stays as defense-in-depth until make ha-up runs confirm zero stuck
untriggered.

Verify

make lint 0 · make build ok · make test 701 pass · functional tests compile. Adds a memory
ClaimWorkflow unit test and a Postgres contract subtest (claims, idempotent for owner, blocks other
live nodes). The real 3-node assertion is the CI e2e tier (can't run a 3-node stack locally).

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