Clean up validate_fast.sh Phase 4 TODOs and remove project-specific references #4

### Keepalive: OFF
<!-- meta:issue:4 -->

### Source Issue #4: Clean up validate_fast.sh Phase 4 TODOs and remove project-specific references

Base: main
Head: codex/issue-4

Source: https://github.com/stranske/Workflows/issues/4

> Topic GUID: 06e94437-09a3-5519-af2f-ee625aada267
> 
> ## Why
> `scripts/validate_fast.sh` contains 12 TODO Phase 4 markers referencing Trend_Model_Project-specific features (SRC_FILES detection, autofix tests, coverage requirements) that are not applicable to this workflow repository. These dead code paths and stale references cause confusion and add maintenance burden.
> Scope
> - `scripts/validate_fast.sh` cleanup of TODO markers and project-specific detection logic.
> 
> ## Tasks
> - [ ] Remove all TODO Phase 4 markers and their associated dead/commented code blocks.
> - [ ] Remove SRC_FILES and AUTOFIX_FILES detection logic that references `src/` directory patterns not present in this repo.
> - [ ] Update linting targets to use `scripts/` and `.github/` instead of `src/ tests/`.
> - [ ] Replace Python package-specific validation (coverage requirements, import tests) with workflow-specific checks (actionlint, YAML validation).
> - [ ] Ensure the script still runs successfully after cleanup.
> 
> ## Acceptance criteria
> - `grep -c "TODO Phase" scripts/validate_fast.sh` returns 0.
> - Script executes without errors: `bash scripts/validate_fast.sh --help` works.
> - No references to `src/` directory or Trend_Model_Project-specific patterns remain.
> - Validation targets are appropriate for a workflow repository (scripts/, .github/).
> 
> ## Implementation notes
> - Keep the core validation framework intact; only remove dead code and update targets.
> - Test with `bash scripts/validate_fast.sh --verbose` to verify functionality after changes.
> - Preserve the intelligent change detection but update the file patterns it looks for.
> 
> ---
> Synced by [workflow run](https://github.com/stranske/Workflows/actions/runs/20290022575).

@codex start

Codex, please derive and propose:
- Scope / key constraints
- Acceptance criteria / definition of done
- Initial task checklist to iterate through

Use the issue details above to draft the plan, then proceed with implementation.

---

Once the plan is posted, comment the snippet below to start execution and enable keepalive:

The following block should be pasted as a separate PR comment:

```markdown
@codex Your objective is to satisfy the **Acceptance Criteria** by completing each **Task** within the defined **Scope**.

**This round you MUST:**
1. Implement actual code or test changes that advance at least one incomplete task toward acceptance.
2. Commit meaningful source code (.py, .yml, .js, etc.)—not just status/docs updates.
3. Mark a task checkbox complete ONLY after verifying the implementation works.
4. **POST A REPLY COMMENT** with completed checkboxes using the **EXACT TEXT** from the lists below.

**CRITICAL - Checkbox Format:**
When posting your reply, copy the **exact checkbox text** from the Tasks and Acceptance Criteria sections below. Do NOT paraphrase or summarize. The automation matches text exactly.

Example format for your reply: