Add code-modernization plugin

Structured workflow (assess → map → extract-rules → reimagine → transform →
harden) and specialist agents (legacy-analyst, business-rules-extractor,
architecture-critic, security-auditor, test-engineer) for modernizing legacy
codebases into current stacks.

plugins/code-modernization/commands/modernize-brief.md

@@ -0,0 +1,60 @@
+---
+description: Generate a phased Modernization Brief — the approved plan that transformation agents will execute against
+argument-hint: <system-dir> [target-stack]
+---
+
+Synthesize everything in `analysis/$1/` into a **Modernization Brief** — the
+single document a steering committee approves and engineering executes.
+
+Target stack: `$2` (if blank, recommend one based on the assessment findings).
+
+Read `analysis/$1/ASSESSMENT.md`, `TOPOLOGY.md`, and `BUSINESS_RULES.md` first.
+If any are missing, say so and stop.
+
+## The Brief
+
+Write `analysis/$1/MODERNIZATION_BRIEF.md`:
+
+### 1. Objective
+One paragraph: from what, to what, why now.
+
+### 2. Target Architecture
+Mermaid C4 Container diagram of the *end state*. Name every service, data
+store, and integration. Below it, a table mapping legacy component → target
+component(s).
+
+### 3. Phased Sequence
+Break the work into 3-6 phases using **strangler-fig ordering** — lowest-risk,
+fewest-dependencies first. For each phase:
+- Scope (which legacy modules, which target services)
+- Entry criteria (what must be true to start)
+- Exit criteria (what tests/metrics prove it's done)
+- Estimated effort (person-weeks, derived from COCOMO + complexity data)
+- Risk level + top 2 risks + mitigation
+
+Render the phases as a Mermaid `gantt` chart.
+
+### 4. Behavior Contract
+List the **P0 behaviors** from BUSINESS_RULES.md that MUST be proven
+equivalent before any phase ships. These become the regression suite.
+
+### 5. Validation Strategy
+State which combination applies: characterization tests, contract tests,
+parallel-run / dual-execution diff, property-based tests, manual UAT.
+Justify per phase.
+
+### 6. Open Questions
+Anything requiring human/SME decision before Phase 1 starts. Each as a
+checkbox the approver must tick.
+
+### 7. Approval Block
+```
+Approved by: ________________  Date: __________
+Approval covers: Phase 1 only | Full plan
+```
+
+## Present
+
+Enter **plan mode** and present a summary of the brief. Do NOT proceed to any
+transformation until the user explicitly approves. This gate is the
+human-in-the-loop control point.