feat(examples): add coding agent harness example

[Agentic-JJ-Web3]

Part of #885. This is the first of three PRs — steps 1–3 of your outline
(harness loop, basic tools, running it with tests and the Burr UI). CLI and
permissions follow separately.

What's here

examples/coding-agent/ — an agent that reads files, writes files, lists a
directory, and runs shell commands, looping until the model answers without
requesting a tool or the step budget runs out.

The graph is human_input → create_prompt → call_llm, with four bound tool
actions fanning out of call_llm and looping back to it. Each tool is bound
separately rather than dispatched from a registry, so the Burr UI shows which
one ran at each step.

Decisions I made without you

You hadn't replied yet, so these are assumptions rather than agreements —
happy to change any of them:

1. Scripted LLM client by default. notebook.ipynb is a required file and
   has to execute, so the example falls back to a client replaying a fixed
   sequence of tool calls when OPENAI_API_KEY isn't set. Real OpenAI client
   when it is.
2. run_bash is not sandboxed. File paths are confined to a workspace
   directory, but a shell command can cd out. The README says so plainly,
   which leaves step 5 something concrete to fix.
3. Four tools, not three — added list_files, since an agent that can't
   see the directory can't orient itself.
4. One tool call per iteration. If the model requests several, the extras
   are dropped. Documented as a known simplification.

Notes

• validate_examples.py passes for coding-agent.
• The tool schema marks only parameters without defaults as required. The
  tool-calling example marks all of them, which misreports optional args —
  happy to open a separate issue for that.
• examples/hello-world-counter crashes at app.visualize() unless the
  Graphviz binary is installed, and passing a stale app_id after clearing
  ~/.burr raises a pydantic error on PointerModel.app_id. Both separate
  from this PR; can file them if useful.

[Agentic-JJ-Web3]

Opened #886 with steps 1–3. I made a few calls in the absence of answers to my questions above — they're listed in the PR description, happy to change any of them.