feat(flows): execute fork branches in parallel with join semantics

[albertoperdomo2]

Summary

Implements the fork/join design from PARALLEL-FLOW-EXECUTION.md. Today the runner walks a single cursor and follows only the first outgoing edge of every node (nextEdge() takes getFlowOutgoingTargets(...)[0]), silently dropping the rest; merge is a pass-through, not a join. A fan-out flow that looks correct in the editor executes only its first branch, and downstream {{steps.<dropped-node>.output}} references resolve to null and fail the run with unknown_template_variable.

Changes

• fork node type — { id, name, type: 'fork', joinNodeId }; joinNodeId explicitly names the paired merge, avoiding ambiguity about which join collects which branches.
• Parallel execution — when the cursor reaches a fork, the runner spawns one execution fiber per outgoing edge and runs them via Promise.all. Each fiber is an independent traversal (own visited set, own step-list copy, stops at the fork's join via stopBeforeNodeId). When all branches arrive, the parent cursor executes the merge once and continues single-cursor from its outgoing edge.
• Per-branch OpenCode sessions — a design-doc deviation, forced by the code: a workspace session executes one prompt at a time (createActiveRunAfterRuntimeStateCheck fails with session_busy on a busy session), so branches sharing the parent session would fail spuriously or interleave. Each branch creates its own session (Flow | <name> | <date> · <branch node>); branch steps are recorded per node as usual, which is what the template context consumes.
• Failure semantics — each fork shares an abort flag: the first branch failure (or termination_unconfirmed) sets it, siblings exit on their next loop iteration, and the fork reports the failure without executing the join. Run cancellation still exits through the existing per-iteration check. Outcome precedence: termination_unconfirmed > failed > cancelled.
• Template context — fiber step lists are merged per nodeId into the parent after the branches settle, so {{steps.<branchNode>.output}} works in post-join nodes; previousOutput resets to null after a fork (branch outputs are referenced explicitly).
• Validation (validateFlowDefinition) — new rules: at most one outgoing edge for nodes other than condition/fork (this is the silent-drop bug, now an error — intentionally flags currently-broken flows); join must reference an existing merge and be unique per fork; ≥2 branches and no direct fork→join edge; no branch dead-ends; no human/slack inside a branch region; no join inputs from outside the region.
• Editor — renaming a join node now remaps fork.joinNodeId (join fields previously fell through unmapped).

Design decisions documented in the OpenSpec change

• The doc's literal fan-in rule ("merge incoming count == fork outgoing count") is replaced by structurally sound containment rules — a condition node inside a branch legitimately converges several rule targets into the join, which breaks the literal count for a correct graph.
• human/slack nodes are rejected inside branches for now: pausing one branch while siblings run needs branch-state persistence and a fork-aware resume path.
• Fork nodes record no run step (the runner intercepts them), so the Prisma FlowNodeType enum and the database need no migration; nodeTypeToPrisma is typed to exclude fork so the compiler keeps that invariant.
• currentNodeId tracks the single cursor only; a retry after a fork failure resumes from the fork and re-runs the region.

OpenSpec

Adds parallel-flow-execution change with proposal, design, tasks, and a flow-execution spec delta (parallel branch execution, join failure semantics, topology validation, backward compatibility). openspec validate parallel-flow-execution --strict passes.

Test plan

• validation.test.ts — well-formed fork accepted (nested forks too); one test per rejection rule; multi-edge rejection
• runner.test.ts — branches run and both outputs render in the post-join template; per-branch session creation; branch failure fails the run without executing the join
• editor-graph.test.ts — join rename remaps joinNodeId
• Full flows suite (27 files, 201 tests) passes; eslint clean on changed files
• tsc --noEmit: no new errors (only the pre-existing tests/providers-* local-env noise, untouched by this diff)
• CI green

Notes / follow-ups

• Editor authoring UI for fork nodes (palette entry, join picker, canvas rendering) is deliberately out of scope; fork flows are expressible via templates and the API.
• ⚠️ fix(flows): keep gateway tokens fresh across flow execution #500 also touches executeFlowNodes (between-step token refresh), so whichever merges second will need a rebase — the changes are complementary, not conflicting in intent.