fix(completion): install the right completions for fish users

[clay-good]

Closes the problem reported in discussion #1197.

What was wrong

openspec completion install detected your shell only from $SHELL, which is your login shell. If you run fish interactively but your login shell is bash (the default on Fedora Silverblue and many container/immutable setups), the command installed bash completions and edited .bashrc — silently doing the wrong thing:

❯ openspec completion install
⠋ Installing bash completion script...
✓ Completion script installed and .bashrc configured successfully

Fish completions were already fully supported — they just never got selected.

What it does

Detection now looks at the parent process (the shell that actually launched openspec) before falling back to $SHELL. So running the command from fish installs fish completions.

• Only a parent that maps to a supported shell (bash/zsh/fish) is trusted; npx, npm scripts, and other non-shell parents fall back to $SHELL exactly as before.
• POSIX-only (ps); Windows/PowerShell detection is untouched.
• Best-effort: any failure reading the parent silently falls back to $SHELL, so nothing regresses.

Proof it works

New unit tests cover the fish-on-bash case, macOS full-path ps output, and every fallback path. Verified end-to-end against the real ps (fish isn't installed locally, so these use bash/zsh to prove the parent shell overrides $SHELL — the exact mechanism a fish user needs):

Parent shell	$SHELL (login)	Detected
bash	/usr/bin/fish	bash ✓ (parent wins)
zsh	/bin/bash	zsh ✓ (mirrors #1197)
node (npx-style)	/bin/bash	bash ✓ (falls back to $SHELL)
bash	/bin/bash	bash ✓ (unchanged)

Notes

• Scope is limited to shell detection. It does not touch the generated completion scripts (that's the separate open PR Fix Fish completions to avoid filesystem fallback #1199).
• Added a patch changeset.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Fixed shell completion installation choosing the wrong shell when the interactive shell differs from the login shell.
  • Improved shell detection by preferring the interactive shell derived from the running parent process, with accurate handling for Fish, Bash, and Zsh (including executable-path cases).
  • Strengthened fallback behavior when parent shell detection fails.
  • Preserved Windows PowerShell detection without running Unix-specific detection.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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Run ID: bfe2819a-2885-432e-806c-c62c9e028fe7

📥 Commits

Reviewing files that changed from the base of the PR and between 7d292d4 and b70be5e.

📒 Files selected for processing (2)

• src/utils/shell-detection.ts
• test/utils/shell-detection.test.ts

🚧 Files skipped from review as they are similar to previous changes (1)

• test/utils/shell-detection.test.ts

---

📝 Walkthrough

Walkthrough

Shell detection now checks the parent process for supported shells before falling back to $SHELL. Tests cover shell parsing, fallback behavior, failures, and Windows handling. A patch changeset documents the update.

Changes

Shell Detection

Layer / File(s)	Summary
Parent-process shell detection src/utils/shell-detection.ts	Maps shell names, inspects the parent process with ps on non-Windows systems, and falls back to $SHELL when needed.
Detection validation and release metadata test/utils/shell-detection.test.ts, .changeset/completion-detect-parent-shell.md	Mocks parent-process inspection, tests shell and fallback cases across platforms, and records a patch release changeset.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant detectShell
  participant execFileSync
  participant Environment
  detectShell->>execFileSync: inspect parent process with ps
  execFileSync-->>detectShell: parent shell name or failure
  detectShell->>Environment: read SHELL when parent detection fails
  Environment-->>detectShell: supported shell value

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Suggested reviewers: tabishb, alfred-openspec

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: fixing completion installation for fish users.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/completion-detect-parent-shell

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/utils/shell-detection.ts`:
- Around line 21-25: Update matchSupportedShell to extract the executable
basename and return a supported shell only when that basename exactly equals
zsh, bash, or fish; do not classify names such as fish-lsp or
bash-language-server. Add a regression test using a fish-lsp parent that
verifies detection falls back to $SHELL.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 80812885-299e-4b76-a42d-4aac350c9659

📥 Commits

Reviewing files that changed from the base of the PR and between 0a99f41 and 7d292d4.

📒 Files selected for processing (3)

• .changeset/completion-detect-parent-shell.md
• src/utils/shell-detection.ts
• test/utils/shell-detection.test.ts

[alfred-openspec]

Verified parent-process precedence, exact basename and login-shell handling, POSIX fallback, and the Windows guard. The focused regression matrix and full CI are green.