fix: recover interrupted installs and correct shell integration

[aooohan]

Fix interrupted-install recovery, Windows MSYS2 zsh PATH serialization, and shell detection through same-name executable shims. Add Fish command and option completion with packaging support and preserve Bash/Zsh completion formats.

Validation: CI and three-platform E2E passed on 563ba7b. Windows tests execute native process/file-lock regressions and MSYS2 zsh; Fish tests run with the real shell. The shim test uses a native process fixture rather than a complete Scoop installation.

Fixes #252
Fixes #490
Fixes #238
Fixes #22

[Copilot]

🟡 Changes recommended

Unresolved Windows zsh PATH serialization/cache issues and empty-version runtime detection remain.

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Pull request overview

This PR adds interrupted-install recovery, fixes Windows/MSYS2 shell integration, and adds Fish completion support.

Changes:

• Adds install markers, locking, recovery logic, and tests.
• Improves shim-based shell detection and zsh PATH handling.
• Adds Fish completion, documentation, packaging, and CI coverage.

File summaries

File	Summary
README.md	Documents Fish completion.
README_CN.md	Documents Fish completion in Chinese.
internal/shell/shell.go	Resolves shells through same-name shims.
internal/shell/open_test.go	Tests shim-aware shell detection.
internal/sdk/sdk.go	Adds install markers and recovery checks.
internal/sdk/install_windows_test.go	Tests Windows locked-payload recovery.
internal/sdk/install_test.go	Tests installation recovery and locking.
internal/sdk/install_lock.go	Provides shared install locking.
internal/sdk/install_lock_windows.go	Implements Windows locking.
internal/sdk/install_lock_unix.go	Implements Unix locking.
internal/env/path_windows_test.go	Tests Windows/MSYS2 PATH handling.
internal/env/env_win.go	Adds zsh PATH serialization.
completions/zsh_autocomplete	Preserves zsh completion formatting.
completions/vfox.fish	Adds Fish command and option completion.
completions/bash_autocomplete	Preserves Bash completion formatting.
cmd/completion.go	Handles CLI completion generation.
cmd/completion_test.go	Tests shell completion integration.
cmd/cmd.go	Registers completion handlers.
AGENTS.md	Documents installation invariants.
.goreleaser.yaml	Packages Fish completion scripts.
.github/workflows/ci.yml	Adds platform-specific regression jobs.

Review details

Suppressed comments (1)

internal/env/env_win.go:33

• The new zsh branch can be skipped entirely by the persistent env cache: envFlag returns cached output for a cached zsh entry before Paths.String() runs, and ConfigState keys that entry only by shell name, not by PATH serialization format or vfox version. Output generated by the pre-change binary can therefore remain semicolon-separated after upgrade, so this MSYS2 fix is not applied until cache invalidation. Invalidate or version the cache when this format changes and cover a pre-existing zsh cache.

	shell := strings.ToLower(os.Getenv(HookFlag))
	if shell == "bash" || shell == "zsh" {

• Files reviewed: 21/21 changed files
• Comments generated: 2
• Review effort level: Lite

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