docs(ci): name the timing fields this forge actually returns

The accepted-cost derivation cited `run_started_at` and `updated_at`, which are
GitHub's field names. This Gitea's runs payload has neither -- it returns
`started_at` and `completed_at`, and omits `run_started_at`, `updated_at` and
`created_at` entirely (confirmed by dumping the keys of a run object). The
figures are unaffected: the script that produced them fell through to the real
fields, so 83 s median over 77-106 s stands.

The derivation was named so a reader could re-take the measurement, and as
written it returned nothing when followed literally, which defeats the only
reason it was there.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LjbhSqQf3pwnPA6MVaWcWL

README.md

@@ -623,7 +623,7 @@ run.** Gitea populates no `changes` field for a title-or-body edit, unlike
 GitHub, so the workflow cannot tell the edit that un-drafts a PR from an
 ordinary body PATCH — a closing-reference fix-up, say. The price is around
 **90 seconds** of a runner shared across four repos on two capacity slots: the
-last twelve non-skipped runs of this job, `run_started_at` to `updated_at` off
+last twelve non-skipped runs of this job, `started_at` to `completed_at` off
 the Actions API, are 83 s median over 77–106 s. `timeout-minutes: 20` is a
 ceiling for a hung suite, not a duration. It is also bounded: the workflow's
 `concurrency:` block groups `pull_request` runs on `github.ref` with