Return WorkflowStartResult from Workflow.start()

[czpython]

T2 of the MCP arc (spec: tmp/mcp-arc-tickets.md). start() now returns WorkflowStartResult(run_id, is_duplicate) so dispatch surfaces can tell a fresh enqueue from the subject's already-live run — the fact the agent-facing dispatch route (T3) needs to report duplicate-as-success. is_duplicate is sourced only from the DBOSQueueDeduplicatedError catch; the terminal-holder recursion path propagates the recursive call's result; no preflight reads, no compat shim. Dispatch callers keep their string contracts via .run_id.

Test evidence: dedup tests now pin both flags (test_duplicate_start_returns_the_live_run, test_duplicate_start_shares_the_run_across_accounts); ~30 assertion sites updated across the four durable test files; ruff + format clean, pyright error multiset identical to main's baseline; backend suite runs in CI.

Merge gate: druks-discovery companion prepared locally on eng/workflow-start-result-companion @ f33f7de (not pushed) — coordinate before merging this.

[czpython]

Closing: cutting is_duplicate removes the whole of this PR. No consumer ever branched on the flag — the agent dispatch reply only formatted a note from it — and it cost a platform-primitive return-type change (start() -> str → a new model) across every caller and ~40 test sites. start() stays a bare run-id string; safe blind dispatch is unchanged (that's DBOS queue dedup, not this field), and the agent dispatch reports one idempotent note.

The removal landed on the two stacked PRs: #36 (0d91b92d) and #37 (88579449). The druks-discovery companion is moot and discarded — with T2 gone, that repo needs no change.