feat: Gate Readiness Assessor (P14) + README update

[antnewman]

Summary

• Adds Gate Readiness Assessor (P14) — synthesises data from all existing assurance modules (P1-P12) into a composite gate-specific readiness assessment
• Supports all 7 IPA review points (Gate 0-5 + PAR) with gate-specific dimension weighting
• Comprehensive README rewrite reflecting the full platform (44 tools, 14 assurance capabilities)
• Developer reference and practitioner guide documentation

How it works

The assessor reads persisted data from P1-P12 modules (does not re-execute them) and scores 8 dimensions:

Dimension	Source Modules	What it measures
Artefact Readiness	P1, P3	Document currency, action closure rate
Data Quality	P2, P4	Compliance trends, AI confidence
Assumption Health	P11	Drift severity, staleness
Governance Maturity	P6, P12	Override patterns, ARMM level
Review Timing	P5	Scheduling urgency signals
Assurance Efficiency	P8	Activity effectiveness
Operational Learning	P7	Lessons capture and sentiment
Complexity Alignment	P10	Domain-appropriate assurance

Readiness levels

• READY: Composite >= 75%, no blocking issues, 4+ dimensions scored
• CONDITIONALLY_READY: Composite >= 50% or insufficient data coverage
• AT_RISK: Composite >= 25%
• NOT_READY: Any CRITICAL signal (severity >= 0.80) or composite < 25%

MCP tools (3 new, 44 total)

• assess_gate_readiness — full assessment with dimension breakdown
• get_gate_readiness_history — track progression over time
• compare_gate_readiness — before/after comparison with deltas

Test plan

• 30 unit tests covering enums, dimension scorers, composite scoring, readiness classification, signal generation, history/comparison, gate weighting, edge cases
• All 30 tests passing
• ruff check clean
• README comprehensively rewritten

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