feat: add retrospective command for spec drift analysis

[emi-dm]

Summary

Adds a new /speckit.retrospective command for post-implementation analysis that measures spec drift - the deviation between what was specified and what was actually built.

Motivation

The Spec-Driven Development workflow currently covers the full lifecycle from specification through implementation, but lacks a critical feedback loop: learning from completed features. Without retrospective analysis, teams lose valuable insights that could improve future specifications and reduce rework.

The Problem

• Teams often don't know how much their implementation diverged from the original spec
• Lessons learned are informal and rarely documented
• Specification quality issues repeat across features
• There's no objective metric for "how well did we spec this feature?"

The Solution

The /speckit.retrospective command provides:

1. Quantified Spec Drift Score - An objective metric showing what percentage of requirements were implemented as specified
2. Root Cause Analysis - Understanding why deviations occurred (spec gaps, tech constraints, scope evolution, etc.)
3. Structured Lessons Learned - Actionable improvements categorized by specification, planning, process, and technical domains
4. Traceability - File-level mapping of what was built vs. what was specified

Expected Impact

• Improved specification quality over time through documented learnings
• Reduced rework by identifying common specification gaps
• Better estimates by understanding where scope typically expands
• Team alignment through shared understanding of what "done" looks like vs. the spec

Design Decisions

• Read-only by default: The command analyzes but doesn't modify files, making it safe to run at any time
• Severity classification: Uses CRITICAL/SIGNIFICANT/MINOR/POSITIVE to prioritize findings (POSITIVE captures improvements over spec)
• Completeness gate: Warns if less than 50% of tasks are complete, as early analysis may be misleading
• Reuses existing script infrastructure: Uses check-prerequisites.sh for consistency with other commands

Usage

Run after implementation is complete (or at a significant milestone):

[Copilot]

Pull request overview

This pull request adds a new /speckit.retrospective command that performs post-implementation analysis to measure spec drift - the deviation between what was specified and what was actually built in a feature. This creates a feedback loop in the Spec-Driven Development workflow by quantifying implementation deviations and generating actionable lessons learned.

Changes:

• Adds a comprehensive retrospective analysis command with read-only operation
• Implements spec drift scoring based on requirement adherence
• Provides structured root cause analysis for deviations with severity classification (CRITICAL/SIGNIFICANT/MINOR/POSITIVE)
• Generates detailed retrospective reports with traceability and actionable recommendations

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