docs: add comprehensive feature, architecture, and requirements documentation (#1089)

New documentation suite in docs/:

- README.md — Documentation index with audience-targeted links
- ARCHITECTURE.md — System architecture: component model, agent patterns,
  data flow, file system layout, installer architecture, hook system,
  runtime abstraction, and design principles
- FEATURES.md — Complete feature reference with 32 features documented,
  each with formal requirements (REQ-*), produced artifacts, process
  descriptions, and functional specifications
- COMMANDS.md — Full command reference: all 37 commands with syntax,
  flags, arguments, prerequisites, produced artifacts, and examples
- CONFIGURATION.md — Complete config schema: 40+ settings across core,
  workflow, planning, parallelization, git branching, gates, safety,
  model profiles, and environment variables
- CLI-TOOLS.md — gsd-tools.cjs programmatic API: all commands across
  state, phase, roadmap, config, verify, template, frontmatter,
  scaffold, init, milestone, and utility categories
- AGENTS.md — All 15 specialized agents: roles, tool permissions,
  spawn patterns, model assignments, produced artifacts, key behaviors,
  and a complete tool permission matrix

Coverage derived from:
- All 37 command files (commands/gsd/*.md)
- All 41 workflow files (get-shit-done/workflows/*.md)
- All 15 agent definitions (agents/*.md)
- All 13 reference documents (get-shit-done/references/*.md)
- Full CLI source (gsd-tools.cjs + 11 lib modules, ~10K lines)
- All 3 hooks (statusline, context-monitor, check-update)
- Installer (bin/install.js, ~3K lines)
- Full CHANGELOG.md (1.0.0 through 1.24.0, ~170 releases)

docs/README.md

@@ -0,0 +1,25 @@
+# GSD Documentation
+
+Comprehensive documentation for the Get Shit Done (GSD) framework — a meta-prompting, context engineering, and spec-driven development system for AI coding agents.
+
+## Documentation Index
+
+| Document | Audience | Description |
+|----------|----------|-------------|
+| [Architecture](ARCHITECTURE.md) | Contributors, advanced users | System architecture, agent model, data flow, and internal design |
+| [Feature Reference](FEATURES.md) | All users | Complete feature and function documentation with requirements |
+| [Command Reference](COMMANDS.md) | All users | Every command with syntax, flags, options, and examples |
+| [Configuration Reference](CONFIGURATION.md) | All users | Full config schema, workflow toggles, model profiles, git branching |
+| [CLI Tools Reference](CLI-TOOLS.md) | Contributors, agent authors | `gsd-tools.cjs` programmatic API for workflows and agents |
+| [Agent Reference](AGENTS.md) | Contributors, advanced users | All 15 specialized agents — roles, tools, spawn patterns |
+| [User Guide](USER-GUIDE.md) | All users | Workflow walkthroughs, troubleshooting, and recovery |
+| [Context Monitor](context-monitor.md) | All users | Context window monitoring hook architecture |
+
+## Quick Links
+
+- **Getting started:** [README](../README.md) → install → `/gsd:new-project`
+- **Full workflow walkthrough:** [User Guide](USER-GUIDE.md)
+- **All commands at a glance:** [Command Reference](COMMANDS.md)
+- **Configuring GSD:** [Configuration Reference](CONFIGURATION.md)
+- **How the system works internally:** [Architecture](ARCHITECTURE.md)
+- **Contributing or extending:** [CLI Tools Reference](CLI-TOOLS.md) + [Agent Reference](AGENTS.md)