chore(#771): convert agent color: hex/magenta values to documented named colors (#823)

* chore(#771): convert agent color: hex/magenta values to documented named colors

Claude Code's sub-agent `color:` field documents only 8 named colors
(red, blue, green, yellow, purple, orange, pink, cyan). Twelve agent
files used hex values and two used the undocumented `magenta`; convert
each to the nearest documented named color so the intended per-agent
TUI color differentiation is spec-compliant.

- agents/*.md: 14 color values hex/magenta -> nearest named color
- scripts/research-profiles.cjs: update the 3 generated research-agent
  profiles (source of truth) so gen-research-agents stays in sync
- docs/AGENTS.md: update documented colors; add missing Color rows for
  gsd-nyquist-auditor, gsd-project-researcher, gsd-phase-researcher
- tests/agent-frontmatter.test.cjs: add regression guard asserting every
  agent color: is in the documented named-color set

Closes #771

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#771): add changeset

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

scripts/research-profiles.cjs

@@ -82,7 +82,7 @@ const PROFILES = [
     name: 'gsd-ai-researcher',
     description:
       'Researches a chosen AI framework\'s official docs to produce implementation-ready guidance — best practices, syntax, core patterns, and pitfalls distilled for the specific use case. Writes the Framework Quick Reference and Implementation Guidance sections of AI-SPEC.md. Spawned by /gsd:ai-integration-phase orchestrator.',
-    color: '"#34D399"',
+    color: 'green',
     tools:
       'Read, Write, Edit, Bash, Grep, Glob, WebFetch, WebSearch, mcp__context7__*',
     requiredIncludes: [
@@ -99,7 +99,7 @@ const PROFILES = [
     name: 'gsd-domain-researcher',
     description:
       'Researches the business domain and real-world application context of the AI system being built. Surfaces domain expert evaluation criteria, industry-specific failure modes, regulatory context, and what "good" looks like for practitioners in this field — before the eval-planner turns it into measurable rubrics. Spawned by /gsd:ai-integration-phase orchestrator.',
-    color: '"#A78BFA"',
+    color: 'purple',
     tools:
       'Read, Write, Edit, Bash, Grep, Glob, WebSearch, WebFetch, mcp__context7__*',
     requiredIncludes: [
@@ -115,7 +115,7 @@ const PROFILES = [
     name: 'gsd-ui-researcher',
     description:
       'Produces UI-SPEC.md design contract for frontend phases. Reads upstream artifacts, detects design system state, asks only unanswered questions. Spawned by /gsd:ui-phase orchestrator.',
-    color: '"#E879F9"',
+    color: 'purple',
     tools:
       'Read, Write, Edit, Bash, Grep, Glob, WebSearch, WebFetch, mcp__context7__*, mcp__firecrawl__*, mcp__exa__*, mcp__tavily__*, mcp__ref__*, mcp__jina__*',
     requiredIncludes: [