fix(crafter): use deterministic filename for AI config collector

[waveywaves]

Summary

• Replace os.CreateTemp (random suffix) with a deterministic filename derived from the full ConfigHash, eliminating duplicate CAS uploads on attestation retries
• Write temp files into a private os.MkdirTemp directory instead of the shared os.TempDir() to prevent symlink attacks
• Extract aiConfigTempFilePath() helper with documentation explaining the root cause
• Add unit tests for filename determinism invariants

Root Cause Analysis

The reviewer questioned whether the filename is actually the cause of duplicates since Materials[m.Name] already uses a deterministic key. The answer is yes — the map key is deterministic, but the Artifact struct inside the material is not.

Here is the chain of causation:

1. uploadAgentConfig() calls os.CreateTemp("", "ai-agent-config-claude-*.json"), which produces a random filename like /tmp/ai-agent-config-claude-3847291.json
2. AddMaterialContractFree() calls addMaterial(), which calls uploadAndCraft()
3. uploadAndCraft() calls fileStats(artifactPath), which does os.Stat(filepath).Name() to get the basename of the temp file
4. That basename becomes Artifact.Name in the material proto (line 136 of materials.go)
5. fileStats() also computes sha256(file_contents) which becomes Artifact.Digest

Because the JSON payload is identical across retries, Artifact.Digest stays the same. But Artifact.Name changes every time (random temp suffix), so the CAS sees a different resource name on each retry. The map key Materials["ai-agent-config-claude"] is overwritten (good), but each retry still triggers a new CAS upload because the artifact metadata differs.

The fix makes the filename deterministic by deriving it from ConfigHash, so Artifact.Name is stable across retries.

Changes

• pkg/attestation/crafter/collector_aiagentconfig.go:

  • Add aiConfigTempFilePath() helper with detailed doc comment explaining the root cause
  • Replace os.CreateTemp + manual write/close with os.MkdirTemp (private dir) + os.WriteFile (deterministic filename)
  • Use full ConfigHash instead of truncated [:12] — no reason to truncate a sha256 hex string
  • Remove now-unnecessary defer os.Remove(tmpFile.Name()) in favor of defer os.RemoveAll(tmpDir)

• pkg/attestation/crafter/collector_aiagentconfig_test.go (new):

  • Test that same (agent, hash) inputs produce identical file paths
  • Test that different config hashes produce different file paths
  • Test that different agent names produce different file paths
  • Test that the full config hash appears in the path (not truncated)

Test Plan

• go build ./pkg/attestation/crafter/ passes
• go test ./pkg/attestation/crafter/... — all 8 sub-packages pass
• New TestAIConfigTempFilePath_Deterministic tests verify:
  • Same config input produces same filename across calls
  • Different config input produces different filenames
  • Full hash is embedded (no truncation)

Fixes #2907