fix: stop modifying shell rc files in postinstall

[jackwener]

Summary

• Remove all .zshrc / .bashrc modification logic from postinstall.js
• Only write completion files to disk (~/.zsh/completions/_opencli, ~/.bash_completion.d/opencli, fish)
• Print clear setup instructions telling the user what to add to their rc file

Motivation

The previous approach parsed .zshrc to find a compinit line and spliced fpath=(...) before it. This broke multi-line shell commands — e.g. zinit blocks where zicompinit matched /compinit/ inside a backslash-continued line, causing the insertion to split the command block and corrupt the user's shell config (#788).

Rather than trying to fix the parser (which can never be fully safe against arbitrary shell syntax), we adopt the approach used by rustup, homebrew, and other CLI tools: write the file, print the instructions, let the user decide where to put it.

Before → After

Shell	Before	After
zsh	Wrote _opencli + modified .zshrc (splice/append)	Writes _opencli + prints fpath/compinit instructions
bash	Wrote opencli + modified .bashrc (append)	Writes opencli + prints source instruction
fish	Wrote opencli.fish (no rc change)	Same — no change

Test plan

• npm install -g prints completion setup instructions for zsh
• .zshrc is not modified
• .bashrc is not modified
• Completion works after user manually adds the printed lines

Closes #788

[jackwener]

Pushed the changes to PR #796 fork branch instead, as requested.