Here's the implementation plan:

The init script (scripts/sw-init.sh) already has zsh and bash completion installation (lines 199-284). The completions directory has all three shell files: _shipwright (zsh), shipwright.bash (bash), and shipwright.fish (fish).

1. Fish shell support — The init script handles zsh and bash but ignores fish entirely
2. Idempotency — The current code always overwrites completion files instead of checking if they're already identical (the issue asks for "already installed" skip messages)
3. Test coverage — Zero tests for the completions section in sw-init-test.sh

• Shell detection via $SHELL and $ZSH_VERSION/$BASH_VERSION
• Zsh: copies to ~/.zsh/completions/, ensures fpath in .zshrc
• Bash: copies to ~/.local/share/bash-completion/completions/, sources in .bashrc
• Success messages with reload instructions

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File	Action	Purpose
scripts/sw-init.sh	Modify	Add fish support, add idempotency checks
scripts/sw-init-test.sh	Modify	Add test cases for shell completion installation

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In the shell detection block (lines 201-213), add fish detection to both the version-variable check and the $SHELL fallback.

Before copying _shipwright, check if the destination file is identical using cmp -s. If identical, show "already installed" and skip.

Same cmp -s pattern as zsh.

After the bash block (line 275), add a new elif for fish:

• Target: ~/.config/fish/completions/shipwright.fish
• Include idempotency check
• No rc file modification needed — fish auto-loads from that directory

Add a fish case (no source needed — fish picks up completions automatically).

• Test bash completions installed to correct path
• Test completions idempotency (second run shows "already installed")
• Test fish completions installed when $SHELL is overridden to fish

Run sw-init-test.sh and npm test to verify no regressions.

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• Task 1: Add fish shell detection ($SHELL == *fish*) to the shell type detection block
• Task 2: Add idempotency check (cmp -s) to zsh completion installation — skip with "already installed" if files match
• Task 3: Add idempotency check (cmp -s) to bash completion installation — skip with "already installed" if files match
• Task 4: Add fish completion installation block — copy shipwright.fish to ~/.config/fish/completions/
• Task 5: Update reload instructions section to handle fish
• Task 6: Add test: bash completions file installed to correct path
• Task 7: Add test: completions idempotency — second run shows "already installed"
• Task 8: Add test: fish completions installed when SHELL is fish
• Task 9: Run sw-init-test.sh — all tests pass
• Task 10: Run npm test — all existing test suites pass

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1. Unit tests in sw-init-test.sh using the existing sandboxed HOME pattern (tests already run with HOME="$TEMP_DIR/home")
2. Shell override: For fish test, override SHELL=/usr/local/bin/fish in the invoke_init call
3. Idempotency test: Run init twice, grep second run's output for "already installed"

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• Fish completions installed automatically during init (in addition to existing zsh/bash)
• Idempotent — running init twice skips with "already installed" message
• Shows clear success/skip messages for all three shells
• Test coverage for completions in sw-init-test.sh
• All existing tests still pass (npm test)
• Bash 3.2 compatible (no associative arrays, no readarray, etc.)