feat(execution-core): emit node_skipped events

[dawidaksamski]

What

A pruned branch previously left no trace: the runner marked the node 'skipped'
internally and moved on, so a node that never ran was indistinguishable from one the
run hadn't reached yet — in the event stream, in the log panel, and on the canvas.

runGraph now emits a node_skipped event for every node whose incoming edges all
resolved without a live route, carrying a reason:

reason	Meaning
branch_not_taken	A predecessor ran and routed elsewhere — the head of the dead branch
upstream_skipped	Every predecessor was itself skipped — deeper inside an already-dead branch

'errorRoute' is covered too: a node that fails and routes to its error handle prunes
the success branch, which reports branch_not_taken.

Notable decisions

• Emitted after the whole wave has propagated, so skips land after that wave's
  node_completed and before the next wave's node_started. Order is a pure function
  of the definition, so replay reproduces it.
• Reason is order-independent. One live-but-pruned incoming edge is enough to make a
  node branch_not_taken, tracked in a set rather than read off the last edge to
  resolve — otherwise a join fed by both a dead branch and a live node that routed
  elsewhere would report differently depending on which predecessor resolved last.
• Emit failures are not swallowed — an exhausted emitEvent fails the run, same as
  execution_started/execution_completed. Unlike node_started there's no node
  execution to route through an error policy. Flagging in case we'd rather a lost skip
  notice never fail an otherwise-good run.
• pathId deliberately left unpopulated — it belongs to the separate
  declared-but-never-emitted contract members question.

AI Studio

Log panel gets a grey badge with the reason inline. Two fixes beyond the planned scope,
both regressions the new status would otherwise have caused: the canvas marker rendered
an empty box for a status none of its branches matched, and highlighting counted skipped
nodes as non-idle, drawing the edge into a pruned branch as if the run had travelled it.