feat(installer): support Zuno and refresh instructions

[sunerpy]

Summary

• add global and project-local Zuno installer support, including safe migration from mcp.codegraph-mcp-server to mcp.codegraph
• make codegraph skill update refresh marker-managed agent instructions while preserving user-owned text
• update the generated CodeGraph instructions to verify index status first and document codegraph_search
• reuse Zuno's standard .agents/skills discovery path to avoid duplicate same-name skills

Validation

• make ci
• cargo build --release -p codegraph-rs
• release-binary smoke test with isolated HOME/XDG_CONFIG_HOME for Zuno dry-run, first update, idempotent second update, and print-config

[codecov]

Codecov Report

❌ Patch coverage is 95.44469% with 21 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/codegraph-cli/src/installer/mod.rs	87.17%	10 Missing ⚠️
...rates/codegraph-cli/src/installer/targets/codex.rs	0.00%	3 Missing ⚠️
...ates/codegraph-cli/src/installer/targets/gemini.rs	0.00%	3 Missing ⚠️
...es/codegraph-cli/src/installer/targets/opencode.rs	0.00%	3 Missing ⚠️
crates/codegraph-cli/src/installer/shared.rs	97.36%	1 Missing ⚠️
crates/codegraph-cli/src/installer/targets/zuno.rs	99.65%	1 Missing ⚠️

@@           Coverage Diff            @@
##             main     #244    +/-   ##
========================================
  Coverage   95.36%   95.37%            
========================================
  Files         141      142     +1     
  Lines       79602    80049   +447     
========================================
+ Hits        75915    76345   +430     
- Misses       3687     3704    +17     

Files with missing lines	Coverage Δ
crates/codegraph-cli/src/installer/registry.rs	98.61% <100.00%> (+<0.01%)	⬆️
...ates/codegraph-cli/src/installer/targets/claude.rs	95.43% <100.00%> (+0.02%)	⬆️
crates/codegraph-cli/src/installer/types.rs	100.00% <100.00%> (ø)
crates/codegraph-cli/src/main.rs	90.88% <ø> (ø)
crates/codegraph-cli/src/installer/shared.rs	95.79% <97.36%> (+0.14%)	⬆️
crates/codegraph-cli/src/installer/targets/zuno.rs	99.65% <99.65%> (ø)
...rates/codegraph-cli/src/installer/targets/codex.rs	96.72% <0.00%> (-0.97%)	⬇️
...ates/codegraph-cli/src/installer/targets/gemini.rs	98.86% <0.00%> (-1.14%)	⬇️
...es/codegraph-cli/src/installer/targets/opencode.rs	98.38% <0.00%> (-0.81%)	⬇️
crates/codegraph-cli/src/installer/mod.rs	93.68% <87.17%> (-0.62%)	⬇️

... and 1 file with indirect coverage changes

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