A build definition says when it runs in a trigger block of its own

`onPush`, `atTimes`, `branches`, and `activeWithin` move into a nested
`trigger`. The split is structural on purpose: the inheritance from
`builds.default` now subtracts one key instead of a list of four, so a
selector added to `TriggerConfig` later is non-inheritable by
construction rather than because someone remembered to extend the list.

A definition still writing those keys flat is refused by name, per file
and scoped like the version check — the machine and project config abort
the start, a branch's committed config fails only that branch. Ignoring
them would leave the build with no trigger at all, which is a job that
quietly stops running: the failure this refusal exists to prevent.

Two more things a definition can now say:

- A `!` prefix in `trigger.branches` excludes, and an exclusion wins
  whatever the order. `["*", "!master"]` gives one branch a build of its
  own without the default build running over it as well — until now the
  only way out of that double build was to drop the second definition's
  push trigger.
- `statusContext` overrides the Gitea check this build reports as, empty
  keeping the repository-wide one. Two builds of a commit shared a
  context and overwrote each other's result, so a quick check beside a
  long build was not readable in Gitea. Pinned like `requirePullRequest`:
  a branch that could pick its context could take over the check a branch
  protection rule depends on.

Fixed on the way: a branch whose builds all belong to named definitions
rendered an empty row in the branches view, reading as "never built"
directly beside its real builds. That row was unreachable before the
exclusion patterns made such a branch possible.

build.gradle.kts

@@ -12,7 +12,7 @@ group = "de.hoennig"
 // bump at least the patch version for every deployment — and only then, not per commit —
 // so the UI footer (BuildProperties), --version and the release notes identify what is
 // actually running; a deployment bundles whatever was committed since the last one
-version = "0.9.19"
+version = "0.9.20"
 
 java {
     toolchain {