Add event sequence defense: sequence unit tests + design review

[dwilding]

The probe agent's integration tests can fail because of event interactions the agent didn't anticipate — e.g. config-changed sets a status message, then pebble-ready fires and clears it. The agent's unit tests pass in isolation (single event) but the integration test fails in CI (full sequence). This caused a false negative in PR #73.

Two mechanisms, providing defense in depth

1. Sequence unit tests (the mechanism)

When an integration test observes a side effect of an event handler, write a companion unit test that fires events in order (chaining ctx.run() calls in the ops testing harness) and asserts on the observable after the full sequence. This runs in run_tox and catches event interaction bugs before CI — the only way run_tox can catch them, since it doesn't run integration tests.

2. Test design review (the reasoning step)

After run_tox passes and before writing .PR.md, do a structured self-review:

1. What event triggers the behaviour you are testing?
2. What observable does your test assert on?
3. List every handler in the charm that modifies that observable.
4. After the trigger fires, will any of those handlers also fire?
5. If a later handler modifies the observable, will your test still pass?

If the answer to #5 is "no" or "not sure," fix the test before proceeding.

Why both?

Either alone is insufficient:

• Sequence test alone: the agent might not write one if it doesn't think to
• Design review alone: the agent might reason incorrectly (as it did in PR All units receive config-changed when application config changes #73)

Together, they provide defense in depth: the review forces the agent to think about the sequence, and the sequence test catches the problem even if the reasoning is wrong.

Changes

• probe_issue.py: rewrote TASK_INSTRUCTIONS with a new "Event sequences and observables" section containing both mechanisms. Updated unit test patterns to show sequence testing with a concrete example. Added the test design review as a mandatory step (step 8) in the workflow.
• AGENT_DESIGN.md: added a new "Event sequences and observables" subsection under the adversarial testing strategy. Updated the prompt composition section to mention sequence testing and the new task instructions content.