docs(temporal): event history limits, secrets and the tool-loop decision

[jimmeryn]

Three documentation items from the Temporal audit. Docs and comments only, no
runtime change.

• What Event History records, and why secrets belong in the worker's environment
  rather than in variables.
• The 2 MB payload and 50 MB history limits, how a chain of N nodes spends them,
  and what an oversize payload actually does.
• The AI agent's tool loop runs inside one activity, so a retry re-runs the whole
  loop. Now owned as a decision instead of read as an accident.

Note

Why READMEs and not the docs site?
There is no Temporal page to put these on.
That page is its own audit item, the "Using Workflow Builder with Temporal"
guide. Limits also belong next to the API they constrain. The one docs-site
change is the variable picker guide, which called variables a place for secrets.

[dawidaksamski]

This will be deprecated once #113 is merged