Call the job's bot account field acts-as

Why

A job names the bot account it acts as with a bot: key:

jobs:
  triage:
    image-file: agent
    bot: coding-agent

bot is the right word for the account. It is what --bot creates, what the directory badge and the profile page say, and what the docs call it. It is a poor name for the field, though: a noun where the job needs a verb, and a word that already means “the account kind” everywhere else in the project. Read aloud, bot: coding-agent says “bot coding agent”, not “this job acts as coding-agent”, so the reference page has to explain in prose what the key should have said.

AGENTS.md asks for the outcome rather than the schema. The outcome is that the job acts as that account.

Scope

• Rename the job field to acts-as:. No second spelling: bot: becomes an unknown key and fails the parse like any other typo.
• Keep bot as the account kind and the label: --bot, the sidebar and directory badge, the kind column, and the Bot accounts page in docs/accounts-and-access.md.
• Update the reference table and the section under it, plus workflows.md, runner.md, and accounts-and-access.md, including the #acts-as anchors.
• Update ci/workflow_test.go and add a case that the old key is rejected.

Acceptance criteria

• acts-as: coding-agent parses and lands on the job’s account name.
• A bad name under acts-as is reported as jobs.<id>.acts-as: ....
• bot: in a job is a parse error.
• No user-facing text calls the field bot:; the account is still a bot.
• ./vole workflow check, the link check, and the test suite pass.

No migration is needed: the field has not shipped in a release and this repository has no tags.