The forge's agent: the lower fence items from #8230 #8255

Part of #8230, item 9. Each one is small:

• steerRun in src/crew/runner.ts: words the person steers into a running turn are not passed to heard(). This fails safe (a write they name stays held), but it should count as the person speaking.
• POST /api/v1/conversation-tokens (src/http/api.ts) has no cap on how many tokens one session may mint, and returns each to the browser for up to an hour. Cap the live tokens per person.
• named() in src/core/fence.ts matches a string value as a substring of the words. A short value (a, 1.0) is named by almost anything. Require a word boundary on both sides.
• A conversation's turns are unbounded. Cap the rows kept per conversation, dropping the oldest.

Each with a test that fails without it.

Closed: already done on main in 57710ffb (pull #8262).