feat(examples): pi-agent livemd — coding agent as CPN (#93)

## Summary

Adds `examples/pi_agent.livemd` modeling a
[pi.dev](https://pi.dev)-style coding agent as a Coloured Petri Net, in
three progressive sections that share state, runtime, and a left/right
Kino UI.

| | Adds | CPN feature |
|---|---|---|
| **A** | minimal ReAct cycle | deferred choice, free output bindings,
self-driving via `action` |
| **B** | risky-tool human gate | union colour set, workitem held in
`:enabled` until human fires |
| **C** | provider race + steering + plan-then-act gate | concurrent
enabled workitems, externally-fed input place, three-way human review |

Section C plan gate exposes three exits — Approve / Reject / Manual
override — each landing a `{:user, _}` line in the dialog so the next
planning round (`MockLLM.proposal_text/2`) quotes the human's note back
into the proposal.

The right column shows the runner's `Occurrence` history live (via
`Storage.occurrences_stream/2`); controls only appear when their target
gate is open and clear when the enactment terminates.

Drive-by lib fix: `ColouredFlow.Definition.Presentation.to_mermaid/1`
switched from the dash arc form (`A --label--> B`) to the pipe form with
quoted labels (`A -->|"label"| B`) so labels containing `[`, `{`, `"`,
etc. — common in CPN inscriptions like `{1, d ++ [tool: tr]}` — render
in mermaid without parser errors. Existing presentation tests updated to
match.

## Test plan

- [x] `mix test test/coloured_flow/definition/presentation_test.exs` (3
tests, 0 failures)
- [x] `mix compile --warnings-as-errors` clean
- [x] Smoke run via `mix run`: A terminates with answer; C runs 13-event
sequence covering propose → manual_input → reject → approve → run →
merge → answer
- [x] End-to-end via Livebook: open `examples/pi_agent.livemd`, all 3
sections render dialog (left top), controls (left bottom, only when
actionable), event history (right); ✅ done footer + cleared controls on
termination
- [x] Mermaid arc labels with brackets / quotes / `<>` render without
"Syntax error in text" iframe

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: fahchen