feat: add git best practices, worktrees, and cost optimization sections

- Add Section 9.12: Git Best Practices & Workflows (~400 lines)
  - Commit message best practices with Conventional Commits
  - Git amend workflow with safety rules
  - Branch management and naming conventions
  - Rewind vs Revert decision tree
  - Comprehensive git worktrees documentation
- Add Section 9.13: Cost Optimization Strategies (~350 lines)
  - Model selection matrix (Haiku/Sonnet/Opus)
  - OpusPlan mode for efficient workflows
  - Token-saving techniques and cost tracking
  - ROI calculations and economic workflows
- Add examples/commands/git-worktree.md slash command template
- Update CHANGELOG.md with v2.0.0 additions
- Update version metadata to 2.0 across documentation

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

examples/commands/git-worktree.md

@@ -0,0 +1,142 @@
+# Git Worktree Setup
+
+Create isolated git worktrees for feature development without switching branches.
+
+**Core principle:** Systematic directory selection + safety verification = reliable isolation.
+
+## Process
+
+1. **Check Existing Directories**: `.worktrees/` or `worktrees/`
+2. **Verify .gitignore**: Ensure worktree dir is ignored
+3. **Create Worktree**: `git worktree add`
+4. **Install Dependencies**: Auto-detect package manager
+5. **Run Baseline Tests**: Verify clean state
+6. **Report Location**: Confirm ready
+
+## Directory Selection
+
+### Priority Order
+
+```bash
+# 1. Check existing directories
+ls -d .worktrees 2>/dev/null     # Preferred (hidden)
+ls -d worktrees 2>/dev/null      # Alternative
+
+# 2. Check CLAUDE.md for preference
+grep -i "worktree.*director" CLAUDE.md 2>/dev/null
+
+# 3. Ask user if neither exists
+```
+
+**If both exist:** `.worktrees/` wins.
+
+## Safety Verification
+
+**For project-local directories:**
+
+```bash
+# Check if directory in .gitignore
+grep -q "^\.worktrees/$" .gitignore || grep -q "^worktrees/$" .gitignore
+```
+
+**If NOT in .gitignore:**
+1. Add line to .gitignore
+2. Commit the change
+3. Proceed with worktree creation
+
+**Why critical:** Prevents accidentally committing worktree contents.
+
+## Creation Steps
+
+```bash
+# 1. Detect project name
+project=$(basename "$(git rev-parse --show-toplevel)")
+
+# 2. Create worktree with new branch
+git worktree add .worktrees/$BRANCH_NAME -b $BRANCH_NAME
+
+# 3. Navigate
+cd .worktrees/$BRANCH_NAME
+```
+
+## Auto-Detect Setup
+
+```bash
+# Node.js
+if [ -f package.json ]; then pnpm install; fi
+
+# Rust
+if [ -f Cargo.toml ]; then cargo build; fi
+
+# Python
+if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+if [ -f pyproject.toml ]; then poetry install; fi
+
+# Go
+if [ -f go.mod ]; then go mod download; fi
+```
+
+## Baseline Verification
+
+```bash
+# Run tests to verify clean state
+pnpm test        # Node.js
+cargo test       # Rust
+pytest           # Python
+go test ./...    # Go
+```
+
+**If tests fail:** Report failures, ask whether to proceed.
+**If tests pass:** Report ready.
+
+## Final Report
+
+```
+Worktree ready at <full-path>
+Tests passing (<N> tests, 0 failures)
+Ready to implement <feature-name>
+```
+
+## Quick Reference
+
+| Situation | Action |
+|-----------|--------|
+| `.worktrees/` exists | Use it (verify .gitignore) |
+| `worktrees/` exists | Use it (verify .gitignore) |
+| Both exist | Use `.worktrees/` |
+| Neither exists | Check CLAUDE.md → Ask user |
+| Not in .gitignore | Add + commit immediately |
+| Tests fail | Report + ask to proceed |
+
+## Common Mistakes
+
+**Skipping .gitignore verification**
+- Worktree contents get tracked, pollute git status
+
+**Assuming directory location**
+- Follow priority: existing > CLAUDE.md > ask
+
+**Proceeding with failing tests**
+- Can't distinguish new bugs from pre-existing
+
+## Cleanup (After Work Complete)
+
+```bash
+# Remove worktree
+git worktree remove .worktrees/$BRANCH_NAME
+
+# Or force if uncommitted changes
+git worktree remove --force .worktrees/$BRANCH_NAME
+
+# Prune stale worktrees
+git worktree prune
+```
+
+## Usage
+
+```
+/git-worktree feature/auth
+/git-worktree fix/session-bug
+```
+
+Branch name: $ARGUMENTS