feat: formalize structural changes signal in decomposition workflow

[rube-de]

Context

PR #122 introduced deterministic size label selection for Epic and New Project sub-issues. Three separate reviewers (across two review rounds) flagged the same gap: the Epic sub-issue sizing table depends on a "Structural Changes" value, but:

1. Epic Flow: The decomposition table schema (Sub-issue | Subsystems | Acceptance criteria | Depends on) has no structural-changes column
2. New Project Flow: Produces MVP features + bootstrap tasks, not a formal decomposition table with subsystems/structural-changes fields

Currently the PM derives structural changes from its assessment during decomposition (WORKFLOWS.md line 211: "Verify each chunk is independently completable — no bundled structural changes"). This works pragmatically but isn't formally recorded.

Proposal

Add an explicit per-sub-issue signal for structural changes. Options:

Option A — Extend decomposition table schema:

| # | Sub-issue | Subsystems | Structural Changes | Acceptance criteria | Depends on |

Option B — Add a "Complexity Hints" block per sub-issue draft:

- Subsystems: Single | Multiple
- Structural changes: Yes | No

Option C — Keep current approach, document it explicitly:
Add a note in WORKFLOWS.md clarifying that the PM determines structural changes during the Round 2 Step 3 verification and carries that assessment forward to sizing.

References

• PR feat: enforce size label selection on Feature/Epic sub-issues #122: feat: enforce size label selection on Feature/Epic sub-issues #122
• Copilot review: feat: enforce size label selection on Feature/Epic sub-issues #122 (review)
• WORKFLOWS.md decomposition table: plugins/project-manager/skills/pm/references/WORKFLOWS.md (line ~216)