Set up cloud dev environment + AGENTS.md

[deyloop]

Summary

Sets up and documents the development environment for this dotfiles repository. This repo has no package manager, lockfile, test suite, or build system — the "application" is the bash utility scripts in scripts/ and the config files (tmux/, vim/, git/, alacritty/, xmonad/, etc.).

Changes:

• Add AGENTS.md with a ## Cursor Cloud specific instructions section documenting what can/can't run in the headless cloud VM, how to lint/validate/run, and setup gotchas (e.g. bash/setup overwrites ~/.bashrc; submodules use SSH URLs).
• Configure the startup update script to install shellcheck (the linter for the bash codebase), non-fatally so it can never break future pods.

Verification

Demonstrated the environment end-to-end (see dev_env_demo.log artifact):

• Lint: shellcheck scripts/* runs (surfaces only pre-existing findings, left untouched).
• Validate configs: git config -f git/config --list, headless vim -u vim/vimrc, and tmux -f tmux/tmux.conf all parse cleanly.
• Run (hello-world): executed scripts/hello, scripts/isosec, scripts/hline, scripts/aln, and the flagship scripts/fmttable Markdown-table formatter — all produce correct output.

dev_env_demo.log

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