Add inshellisense IDE-style autocomplete to shared shell profile

[josephschmitt]

Wire up microsoft/inshellisense across all three shells (bash, zsh, fish).
inshellisense auto-starts a session that wraps the interactive shell, so the
source line must be the last command in each rc file — it's appended after the
profile-override loops in .bashrc, .zshrc, and fish/config.fish.

• Gate the block behind is_integrated_terminal so it's skipped in VS Code /
  Neovim / Emacs / Zed terminals, mirroring auto_start_tmux.
• Source ~/.inshellisense/init//init. only if it exists, so shells
  are unaffected until is init is run and the non-interactive CI startup-time
  harness stays inert (inshellisense is intentionally not installed in CI, where
  a wrapping PTY session would hang the measurement).
• Declare the is binary for macOS in nix-darwin systemPackages; npm install is
  documented for Ubuntu / remote sandboxes.
• Document the integration (must-be-last, editor-terminal gating, provisioning)
  in the shell README, and cross-reference from the root, fish, and nix-darwin
  READMEs.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01RKQHwwanmxtTm74iM6SdH7

Greptile Summary

This PR wires up inshellisense IDE-style autocomplete across bash, zsh, and fish by appending an init block to each shell's RC file as the final statement.

• The block is guarded by a file-existence check (~/.inshellisense/init/<shell>/init.<ext>) so shells stay unaffected on unprovisioned machines, and by is_integrated_terminal so it is skipped in VS Code, Neovim, Emacs, and Zed terminals — matching the existing auto_start_tmux pattern.
• The inshellisense package is declared in darwin.nix for macOS; npm installation is documented for Ubuntu/remote sandboxes.
• Documentation in the shell README clearly explains the must-be-last constraint, the recursion guard (ISTERM) handled by the upstream init file, and warns users not to re-append the source line via is init <shell> >> ~/.<shell>rc.

Confidence Score: 5/5

Safe to merge — the changes are additive shell startup snippets with proper existence and editor-terminal guards, and a single nixpkgs package addition that resolves correctly.

All three shell integrations follow the established pattern in the repo: file-existence guard prevents breakage on unprovisioned machines, is_integrated_terminal correctly gates out editor terminals, and the must-be-last placement is consistent with how inshellisense works. The inshellisense nixpkgs attribute name is valid.

No files require special attention.

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