refactor: convert scaffold templates to claude-native format with unified mode dispatch

[l50]

Key Changes:

• Replaced Go template conditional blocks ({{if eq .Mode ...}}) with a single unified prompt that dispatches mode at runtime via natural language keywords, making the scaffolded files compatible with Claude Code's native agent format
• Added YAML frontmatter to system.md.tmpl so generated agents work directly as Claude Code subagents via symlink
• Consolidated duplicate edit/readonly rule sets into a single document with clearly labeled mode-specific subsections, reducing template size by ~35%
• Added a test asserting that rendered output opens with YAML frontmatter and contains no residual template syntax

Added:

• YAML frontmatter block in system.md.tmpl — includes name, description, tools, and model fields so the generated system.md doubles as a Claude Code agent definition without modification
• TestRenderClaudeNativeOutput test in scaffold/templates_test.go — verifies frontmatter presence, agent name field, and absence of {{ in all three rendered template outputs

Changed:

• Mode dispatch strategy across all templates — replaced compile-time {{if eq .Mode "edit"}} / {{if eq .Mode "readonly"}} branching with runtime keyword detection ("readonly", "report only", "analysis only", "do not modify"), so a single rendered file handles both modes
• system.md.tmpl hard rules — merged the two separate numbered lists into one document with ## Edit-mode rules and ## Readonly-mode rules subsections; trimmed redundant phrasing and consolidated rules 10 and 17–19 for concision
• system.md.tmpl workflow phases — collapsed the duplicated Phase 3 and Phase 4 blocks into a single shared sequence with inline mode annotations; removed the redundant "Read after writing" step in favor of trusting the Edit tool's output
• system.md.tmpl output format — converted top-level ## report sections to ###/#### headings nested under ## Edit-mode report and ## Readonly-mode report headers; inlined "What was changed" / "Why" and "What is wrong" / "Suggested fix" labels to reduce vertical space
• agent.md.tmpl — rewrote as a single unified execution rules block with "Edit mode only" and "Readonly mode only" subsections; collapsed output compliance into a single paragraph referencing system.md rather than duplicating the full section list
• task.md.tmpl — merged the two conditional task prompts into one paragraph that states the default (fix in place) and the opt-in keyword for readonly; removed the trailing {{end}} blocks
• README.md.tmpl — updated mode description from "conditional prompts via Go template syntax" to "dispatched at runtime" and added the Claude Code symlink instruction with trigger keyword examples

Removed:

• All {{if eq .Mode "edit"}} / {{if eq .Mode "readonly"}} / {{end}} conditional blocks from all four templates — mode selection is now entirely prose-driven at runtime
• Duplicate rule sets — the standalone readonly HARD RULES section and the standalone edit HARD RULES section are replaced by the unified two-subsection layout
• Separate Code to review and fix: / Code to analyze (read-only): INPUT labels — replaced with a single label that describes both modes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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