fix(windows): preserve source extension when copying CLI to versioned dir

[AgentA-asaf]

Summary

• Root cause: cf1710f hardcoded .cmd as the versioned bin destination on Windows to fix npm-based providers (codex/gemini). This broke full-package installs: claude.exe was copied into claude.cmd — a PE binary with a batch filename — causing cmd.exe /C to fail and get_cli_version to return (unknown).
• Fix: Step 1 (already-installed check) now probes both .exe and .cmd candidates. Fast-path copy derives the destination extension from the source file (.exe→.exe, .cmd→.cmd). Slow-path (post-npm-install) still uses .cmd since npm always produces .cmd wrappers on Windows.
• Retro doc: docs/retro-cli-exe-cmd-regression.md

Affected providers

Provider	Install method	Before	After
claude	Full package installer	.exe copied as .cmd ❌	.exe copied as .exe ✅
codex	npm install	.cmd → .cmd ✅	unchanged ✅
gemini	npm install	.cmd → .cmd ✅	unchanged ✅

Test plan

• Clear ~/.agentmux/<version>/cli/claude/ and relaunch — should detect ~/.local/bin/claude.exe and copy as claude.exe, not claude.cmd
• Version should show real version string, not (unknown)
• Auth flow should work end-to-end
• Codex/Gemini npm install path unchanged (still produces .cmd)

🤖 Generated with Claude Code

[reagentx-workflow]

ReAgent Diagnostics

Field	Value
ReAgent Version	5.12.4
Project Context	CLAUDE.md loaded
Model	claude-opus-4-6
Effort	high
Ref Repos	Disabled
Merge Analysis	Clean
Review Time	125.6s
Timestamp	2026-03-28T21:12:57Z
Repository	agentmuxai/agentmux
PR	#249

---

Issues:

• agentmuxsrv-rs/src/server/websocket.rs:1733 - installsysdep timeout (5 min) drops the child without killing it; tokio's Child::drop detaches but does not kill the process, so a timed-out winget/brew installer will continue running as an orphan. Need to child.kill() before returning the timeout error.
• Cargo.toml:7-8 - Unrelated release profile changes (lto = true → "thin", codegen-units = 1 → 4) not mentioned in PR description. These weaken release binary optimization for faster compile times — should be a separate commit/PR or at least documented.

[reagentx-workflow]

ReAgent Diagnostics

Field	Value
ReAgent Version	5.12.4
Project Context	CLAUDE.md loaded
Model	claude-opus-4-6
Effort	high
Ref Repos	Enabled (dev-tools, shared-infrastructure)
Merge Analysis	Clean
Review Time	134.5s
Timestamp	2026-03-28T21:44:39Z
Repository	agentmuxai/agentmux
PR	#249

---

Issues:

• agentmuxsrv-rs/src/server/websocket.rs:1778 - installsysdep timeout (5 min) drops the future containing child without killing it; tokio::process::Child::drop detaches but does not kill the process, so a timed-out winget/brew installer continues running as an orphan. Call child.kill() before returning the timeout error.
• Cargo.toml:7-8 - Unrelated release profile changes (lto = true → "thin", codegen-units = 1 → 4) not mentioned in PR description. These weaken release binary optimization — should be a separate commit/PR or documented.
• src-tauri/src/commands/platform.rs:341 - stdout reader task breaks after capturing the OAuth URL, dropping the pipe handle. On Linux/macOS the CLI process receives SIGPIPE on its next stdout write, which can terminate the login process before the OAuth callback completes. Continue draining the pipe after URL capture instead of breaking.
• src-tauri/src/commands/platform.rs:356 - Same pipe-drop issue on the stderr reader task — break after URL capture can kill the CLI via SIGPIPE on subsequent stderr writes.

[reagentx-workflow]

ReAgent Diagnostics

Field	Value
ReAgent Version	5.12.4
Project Context	CLAUDE.md loaded
Model	claude-opus-4-6
Effort	high
Ref Repos	Disabled
Merge Analysis	22 regression(s) detected
Review Time	90.6s
Timestamp	2026-03-29T04:02:46Z
Repository	agentmuxai/agentmux
PR	#249

---

Issues:

• agentmuxsrv-rs/src/server/websocket.rs:1778 - installsysdep timeout drops the future containing child without killing it; tokio::process::Child::drop detaches but does not kill the process, so a timed-out winget/brew installer continues running as an orphan. Call child.kill().await before returning the timeout error.
• src-tauri/src/commands/platform.rs:341 - stdout reader task breaks after capturing the OAuth URL, dropping the pipe. On Linux/macOS the CLI process receives SIGPIPE on its next stdout write, which can terminate the login process before the OAuth redirect callback completes. Continue draining the pipe after capturing the URL instead of breaking.
• src-tauri/src/commands/platform.rs:356 - Same SIGPIPE issue on stderr reader — break after URL capture drops the pipe and can kill the CLI via SIGPIPE on subsequent writes.
• Cargo.toml:7-8 - Unrelated release profile changes (lto = true → "thin", codegen-units = 1 → 4) weaken release binary optimization; not mentioned in PR description and should be a separate commit or documented.