feat(vscode): auto-manage remote.SSH.path on macOS + friendly no-destination usage

[DevSecNinja]

Description

Two follow-ups on top of #546 (VS Code token forwarding). Stacked on feat/copilot-cli-vscode — review/merge #546 first (base will collapse to main once #546 merges).

1. Removes the manual VS Code step on macOS — a chezmoi modify_ script sets remote.SSH.path in ~/Library/Application Support/Code/User/settings.json automatically on chezmoi apply.
2. Friendlier copilot-ssh / copilot_ssh — a clear usage message when called with no destination, instead of ssh's raw usage dump.

Changes

• home/Library/Application Support/Code/User/modify_settings.json.tmpl — chezmoi modify_ script (macOS-scoped via .chezmoiignore). Safety-first:
  • sets only remote.SSH.path; all other settings preserved;
  • byte-for-byte no-op (incl. trailing newline) when the value already matches — zero churn;
  • never rewrites JSONC (comments / trailing commas that jq can't parse) — leaves the file untouched and prints a manual hint.
• copilot-ssh.sh / copilot_ssh.fish — friendly "no destination given" usage message + non-zero exit.
• docs/copilot-cli.md — document the auto-managed macOS setting and the manual path for other OSes.
• tests — modify-vscode-settings.bats (create / add-preserving / no-churn / JSONC-safe / update) and a no-destination case in copilot-ssh.bats.

Notes

• Scoped to macOS (the only desktop OS in use); other OSes keep the documented manual remote.SSH.path step.
• Raw echo/printf are intentional here (the modify_ script's stdout is the settings file; consistent with the merged helpers) rather than log.sh.

Checklist

• Linting passes locally (shellcheck/shfmt on changed files; fish -n)
• Commit messages follow Conventional Commits
• Documentation updated (docs/copilot-cli.md)

Not urgent — for review alongside #546 once you're happy with copilot_ssh stability.