A build definition carries the whole build; branches is legacy

`builds` and the legacy `branches` are now either/or: `branches` is read
only while the merged configuration defines no build at all — a leftover
`builds.maxConcurrent` is not one — and ignored with a warning as soon as
one exists. Two half-answers to "what does this build run" would silently
pull against each other, and the committed configs still carrying both
must not change behaviour before they are migrated.

A definition therefore gained the settings it was missing:
`requirePullRequest` and `docker.enabled`/`network`. Those stay pinned —
`stripPinned` now removes them from a branch layer wherever they appear,
in a definition as well as in a legacy branch entry.

`builds.default` becomes the base every other definition inherits its
settings from, never its trigger: `onPush`, `atTimes`, `branches`, and
`activeWithin` say when and where *this* build runs. The inheritance is
applied after all layers are merged, which is what makes a build invented
on a branch inherit the host's sandbox policy instead of the data-class
default — otherwise a branch could get a native build past the pinning by
defining a job the host has never heard of.

Two bugs found on the way, both the same shape as the build command the
artifact page used to get wrong:

- `FileArtifactStore` read the artifact directories from the plain branch
  settings, so a job adding its own `artifactDirs` never had them stored.
  It goes through `GitTallyConfig.buildSettings` now, like everything else
  that asks what a build runs.
- `Watcher.definitionsFor` cached the per-branch definitions by head
  commit alone, so an edited machine or project config only took effect
  once the branch moved — on a quiet branch, never. The primary config is
  part of the cache key now.

AGENTS.md

@@ -40,6 +40,8 @@ All production code lives under `de.hoennig.gittally`, with sub-packages `comman
 - When config keys change, three places must stay in sync: the `GitTallyConfig` data classes, the `InitCommand` templates, and `docs/configuration.md`.
 - Every config file may declare `gitTally.version.since`/`below` (the GitTally it is written for, never a format version — no API is involved). `since` is enforced in both directions, using `ConfigVersions.FORMAT_BROKE_IN` for "file predates a breaking change"; `below` only warns. A violation aborts the start for the machine and project config, but fails only that branch's builds for a branch config.
 - A branch describes its own CI: its committed `.gittally.yml` is the branch layer (`ConfigLoader.loadWithBranchLayer`, used by the watcher per origin branch and by `loadForWorktree` at build time) and takes precedence over `.git`/project — including the whole `builds` section, so a new configuration can be tried out on a branch without affecting other branches. Only the pinned set is stripped from that layer: secrets (`git`), host/repository sections (`server`, `gitea`, `executor`, `watcher`), the docker sandbox policy (`docker.enabled`, `docker.network`), and the trust gate (`requirePullRequest`). A branch must never reach credentials, disable its container, change its network, raise global concurrency, or bypass its own pull-request gate; a branch's definitions apply to that branch alone.
+- A build definition carries the complete description of its build. `builds.default` is the base every other definition inherits its settings — never its trigger (`onPush`, `atTimes`, `branches`, `activeWithin`) — from, and the inheritance is applied after all layers are merged: that order is what makes a build a branch invents inherit the host's sandbox policy instead of the data-class default, so the pinning also holds for a build the host has never heard of.
+- `builds` or the legacy `branches`, never both: `branches` is read only while the merged config defines no build at all (`builds.maxConcurrent` is not one), and ignored with a warning as soon as one exists. The section is deprecated and goes away once the repositories have migrated; then `ConfigVersions.FORMAT_BROKE_IN` gets set and a leftover `branches:` key must be rejected by name — the version check alone cannot catch a file that declares no version.
 - Web UI: server-rendered Thymeleaf plus one hand-written `static/gittally.js` — no SPA framework, no frontend build pipeline; every fetch has a timeout and an explicit error badge; `UiFormats` and `gittally.js` must produce identical display formats.
 - Git and Docker access shells out to the CLIs (`GitCommandRunner`, `docker`) — no JGit, no Docker SDK.