feat: inject project history context into extend-mode template generation

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Summary

• In extend mode, assembles HISTORY_CONTEXT from .mgw/project.json (milestone names, issue titles, pipeline stages) and gsd-tools history-digest
• Injects context as <project_history> block in generate_template AI prompt with explicit instruction not to re-suggest already-built features
• Syncs .claude/commands/mgw/project.md which was missing the entire extend-mode gather_inputs block

Closes #58

Changes

• commands/project.md — gather_inputs extend-mode block: added HISTORY_CONTEXT assembly; generate_template: added <project_history> injection + anti-duplication instruction
• .claude/commands/mgw/project.md — same edits + added missing extend-mode gather_inputs block (deployed copy was out of sync)

Test Plan

• Run /mgw:project in extend mode on a project with completed milestones — verify generated template references previously-built features and doesn't re-suggest them
• Run with no .mgw/project.json — HISTORY_CONTEXT should fall back to "No prior history available."
• Run where gsd-tools is unavailable — GSD_DIGEST should default to empty string without error

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Testing Procedures

The following steps verify the three scenarios described in the test plan.

Setup

# From a project directory that uses MGW
cd <your-project-dir>

Test 1: Extend mode with existing milestones

# Ensure .mgw/project.json exists with at least one milestone
cat .mgw/project.json | jq '.milestones'

# Run project command in extend mode
# (choose "extend" when prompted for mode)
/mgw:project

# Expected: generated template references previously-built milestone features
# Expected: AI does NOT suggest re-implementing already-completed items

Test 2: Fallback when no project.json exists

# Temporarily rename or remove project.json
mv .mgw/project.json .mgw/project.json.bak

# Run project command in extend mode
/mgw:project

# Expected: HISTORY_CONTEXT defaults to "No prior history available."
# Expected: command completes without error

# Restore
mv .mgw/project.json.bak .mgw/project.json

Test 3: Graceful fallback when gsd-tools is unavailable

# Temporarily shadow gsd-tools with a broken path
PATH_BACKUP=$PATH
export PATH=$(echo "$PATH" | tr ':' '\n' | grep -v gsd-tools | tr '\n' ':')

# Run project command in extend mode
/mgw:project

# Expected: GSD_DIGEST defaults to empty string
# Expected: no error about missing gsd-tools; HISTORY_CONTEXT still assembled from project.json alone

# Restore PATH
export PATH=$PATH_BACKUP

Pass Criteria

• Test 1: Template content avoids duplicating features from project history
• Test 2: "No prior history available." appears in the AI prompt when project.json is absent
• Test 3: Command completes successfully with gsd-tools absent; no unhandled errors