The branch config takes precedence, including its build definitions

A branch's committed .gittally.yml describes that branch's CI, so it wins
over the project and repo-install config — the `builds` section included.
Pinning it was wrong: a new build definition can only be tried out by
committing it on a branch, and pinned it neither took effect at build time
nor existed for the watcher, so the job silently never ran.

The watcher now decides per branch from that branch's own definitions,
reading its committed config via `git show` and caching it by head commit,
so the read happens only when the branch moved; an unreadable config falls
back to the primary definitions instead of failing the poll cycle. A
branch's definitions are evaluated for that branch alone, so a definition
committed on one branch can never trigger builds of another.

The pinned set is reduced to what does not describe this branch's build:
secrets (`git`), the host and repository sections (`server`, `gitea`,
`executor`, `watcher`), the sandbox policy (`docker.enabled`/`network`),
and the trust gate (`requirePullRequest`). Letting a branch set its own
build command through a definition grants no new power — `branches.*.
buildCommand` always allowed exactly that — while the sandbox and the gate
decide whether untrusted branch code runs on the host at all.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/main/kotlin/de/hoennig/gittally/git/GitService.kt

@@ -258,6 +258,19 @@ class GitService(
         return if (result.isSuccess) result.stdout.trim() else null
     }
 
+    /**
+     * The content of [path] as committed in [commit], or null when that commit has no
+     * such file — used to read a branch's committed `.gittally.yml` without a worktree.
+     */
+    fun showFileAtCommit(
+        commit: String,
+        path: String,
+        workingDir: Path = Paths.get("."),
+    ): String? {
+        val result = runner.run(listOf("git", "show", "$commit:$path"), workingDir)
+        return if (result.isSuccess) result.stdout else null
+    }
+
     fun headCommit(workingDir: Path = Paths.get(".")): String =
         runner
             .runOrThrow(listOf("git", "rev-parse", "HEAD"), workingDir)