fix: enforce repomap token cap and fix install.ps1 file lock on Windows

[batalabs]

Summary

• Repomap overflow: repomap.Format() was ignoring MaxTokens=2000 config. After recent file additions the repo map grew to ~38k tokens, overflowing glm-5.1's practical API limit on every request and causing muxd to not respond on new sessions. Now enforces the cap by truncating at the nearest newline with a ... (truncated) notice.
• install.ps1 file lock: Move-Item failed with "file already exists" when muxd.exe was running (Windows locks executables). Fixed with stop-process + rename-old + move-new + delete-old pattern.

Test plan

• Build and run muxd — verify new sessions respond normally
• Verify repo map in system prompt is ≤2000 tokens
• Run install.ps1 while muxd is running — should stop the process and complete successfully