feat(builds): reuse the image of an unchanged build #404

Problem

Every deployment builds every Git source again, even when its commit and build settings haven't changed: app deploy, an edit that deploys, a push and a preview all resolve their sources from scratch. Changing a variable costs a full build, and once builds run on a build machine (#403) that may be off, it can't deploy at all.

Proposed solution

A build is identified by its BuildInputs: the repository, the commit, and the rendered source.build.* settings, build arguments included.

• Reuse: the deployment checks the repository out as today, which resolves the commit. If a build of this installation succeeded with the same inputs and its image is still present, the deployment uses that image instead of building.
• The build history shows it: a reused build is recorded as one, naming the build it reuses, so the Builds page and piquelctl builds list explain why nothing was built.
• Forcing a build: --rebuild on the commands that deploy, and the same choice in the dashboard's deploy dialog. A rebuild pulls base images again.

Reuse applies across environments of an installation, so a preview and production building the same commit build it once.

Changes in behaviour

A redeployment of an unchanged commit no longer refreshes base images. docs/user/applications/git-builds.md ("How it builds") says it does, and needs updating.

Related: #329 (services sharing one build) needs the same lookup. Part of #403.

Written with Claude Fable 5.2 in Claude Code (via T3 Code).