Add Artifact Regeneration Support

Problem

Currently, there is no way to regenerate artifacts in the OPSX workflow:

• /opsx:apply just reads whatever's on disk
• /opsx:continue only creates the NEXT artifact - won't touch existing ones

If you edit design.md after tasks.md exists, your only options are:

1. Delete tasks.md manually, then run /opsx:continue
2. Edit tasks.md manually

The documentation claims you can "update artifacts mid-flight and continue" but there's no mechanism that actually supports this.

Proposed Solution

Two parts:

Part 1: Staleness Detection

Add artifact staleness detection to /opsx:apply:

1. Track modification times: When generating an artifact, record the mtime of its dependencies
2. Detect staleness: When /opsx:apply runs, check if upstream artifacts (design.md, specs) have been modified since tasks.md was generated
3. Prompt user: If stale, ask: "Design was modified after tasks were generated. Would you like to regenerate tasks with /opsx:continue?"

User Experience

Vision: Seamless Mid-Flight Correction

This is the workflow we want to enable (currently documented but not supported):

You: /opsx:apply

AI:  Working through tasks...
     ✓ Task 1.1: Created caching layer
     ✓ Task 1.2: Added cache invalidation

     Working on 1.3: Implement TTL...
     I noticed the design assumes Redis, but your project uses
     in-memory caching. Should I update the design?

You: Yes, update it to use the existing cache module.

AI:  Updated design.md to use CacheManager from src/cache/
     Updated tasks.md with revised implementation steps
     Continuing implementation...
     ✓ Task 1.3: Implemented TTL using CacheManager
     ...

No restart needed. Just update the artifact and continue.

Staleness Warning UX

When user manually edits an upstream artifact:

$ /opsx:apply

⚠️  Detected changes to upstream artifacts:
    - design.md modified 5 minutes ago (after tasks.md was generated)

Options:
1. Regenerate tasks (recommended)
2. Continue anyway with current tasks
3. Cancel

>

Part 2: Regeneration Capability

Add a way to regenerate specific artifacts:

# Option A: Flag on continue
/opsx:continue --regenerate tasks

# Option B: Separate command
/opsx:regenerate tasks

# Option C: Interactive prompt when staleness detected
/opsx:apply
# "Design changed. Regenerate tasks? [y/N]"

Technical Approach

Option A: Metadata File

Store .openspec-meta.json in change directory:

{
  "tasks.md": {
    "generated_at": "2025-01-24T10:00:00Z",
    "dependencies": {
      "design.md": "2025-01-24T09:55:00Z",
      "specs/feature/spec.md": "2025-01-24T09:50:00Z"
    }
  }
}

Option B: Frontmatter

Add YAML frontmatter to generated artifacts:

---
generated_at: 2025-01-24T10:00:00Z
depends_on:
  - design.md@2025-01-24T09:55:00Z
---
# Tasks
...

Option C: Git-based

Use git to detect if upstream files changed since downstream was last modified. No extra metadata needed but requires git.

Non-Goals

• Automatic regeneration (user should always choose)
• Blocking apply entirely (just warn)
• Tracking code file changes (only artifact dependencies)

Dependencies

• Should be implemented after fix-midflight-update-docs so docs are accurate first
• Could be combined with that change if desired

Success Criteria

• User is warned when applying with stale artifacts
• Clear path to regenerate if needed
• No false positives (only warn when genuinely stale)
• Documentation claims become actually true