Incomplete artifacts marked as "done" after workflow interruption

[jiehu03]

Problem

When a workflow (/opsx:propose, /opsx:ff, etc.) is interrupted mid-execution, partially-written artifact files remain on disk. On re-execution, the system detects these files and marks the artifacts as status: "done", causing the workflow to skip them — leaving the change in an inconsistent state with incomplete artifacts that cascade into poor downstream output.

Root Cause

detectCompleted() in src/core/artifact-graph/state.ts uses artifactOutputExists() which only checks file existence (fs.statSync / glob match). No content validation is performed — an empty or half-written file is treated identically to a complete one.

Reproduction

1. Run /opsx:propose "add-dark-mode"
2. Interrupt after proposal.md is created but before content is fully written
3. Re-run /opsx:continue on the same change
4. openspec status --json reports proposal as status: "done" → workflow skips it

Impact

• Stuck workflows — no clean way to resume after interruption
• Cascading quality degradation — downstream artifacts built on incomplete context
• Silent failure — no warning or error is produced
• Only workaround is manually finding and deleting the incomplete files

Affected Code

• src/core/artifact-graph/state.ts — detectCompleted(), isArtifactComplete()
• src/core/artifact-graph/outputs.ts — artifactOutputExists(), resolveArtifactOutputs()

[mvanhorn]

Submitted PR #1098. Adds artifactOutputContentValid + artifactOutputComplete in outputs.ts; isArtifactComplete now requires non-blank, non-HTML-comment content. 7 new tests cover empty / whitespace / comment-only / valid / glob-mixed paths.