Step 05: Artifact Store

Prerequisites: steps 01, 04. Read README.md and 00-legacy-analysis.md first.

Goal

Persist build artifacts (logs plus configured report directories) with stable naming and retention pruning.

Create package de.hoennig.gittally.artifacts:

ArtifactStore service implementing the interface stubbed in step 04.
Artifact root: a configurable directory (new key artifacts.rootDir), defaulting to ${XDG_STATE_HOME:-~/.local/state}/gittally/artifacts/<repo-key>. Do NOT default to /tmp like legacy — artifacts vanished on reboot.
Repo key: sanitized absolute repo path (legacy repository_key): non [A-Za-z0-9._-] → _.
Artifact key per build: sanitized branch name + 12-char SHA-256 prefix, plus sanitized start timestamp + hash (legacy build_artifact_key); keep this scheme so URLs stay predictable.
persist(build, stagingDir): copy configured artifactDirs and the log files into staging, then atomically move staging → <root>/branches/<artifactKey>/.
prune(keptResults): delete artifact directories whose keys are no longer in the result repository (call after repository retention pruning from step 01).
Provide artifactDir(artifactKey) lookups for the server (step 07).

New key artifacts.rootDir (empty = platform default above). Update GitTallyConfig, InitCommand templates, and docs/configuration.md together.

Key naming: sanitization, hash stability, collision of similar branch names.
Persist: copies configured dirs, missing artifact dirs are skipped with a log line, atomic move (no partial dirs on failure).
Prune: deletes exactly the unreferenced dirs, never anything outside the artifact root.

./gradlew ktlintFormat then ./gradlew build is green.
Step 04's executor persists artifacts through the real store (integration test).