fix: manage ~/.vimrc as a bounded block in user-owned config

[jiminu]

Summary

Changes user-vimrc from a managed symlink at ${XDG_CONFIG_HOME:-$HOME/.config}/vim/vimrc to a bounded block inside $HOME/.vimrc.

Context & Motivation

• Vim support for $XDG_CONFIG_HOME/vim/vimrc was introduced in patch 9.1.0327.
• Ubuntu 24.04 LTS (Selfishell's primary Linux target) packages Vim 9.1.0016, which does not search the XDG path. As a result, minimal profile installations on Ubuntu 24.04 (and macOS environments with standard system Vim) did not read the managed vimrc.
• Furthermore, treating ~/.vimrc as a user-owned configuration file containing a bounded Selfishell block aligns with Selfishell's design contract for .zshrc, .zprofile, and config.ghostty: preserving existing user configuration and allowing custom keymaps/plugins outside the block.

Changes

1. Resource declaration: Changed user-vimrc from link at ~/.config/vim/vimrc to block at $HOME/.vimrc in lib/resources.sh.
2. Block definition & Vim syntax: Defined user-vimrc block with " comments in lib/managed.sh and extended block marker generation/inspection to support custom comment characters.
3. Preflight: Added user-vimrc preflights to lib/commands/install.sh and lib/commands/update.sh.
4. Documentation: Updated AGENTS.md user-owned configuration section.
5. Tests & E2E:
   • Updated tests/managed_install_test.bash and scripts/macos-configuration-e2e.sh to verify ~/.vimrc block lifecycle, idempotency, update, restoration, and symlink rejection.
   • Added standard startup Vim regression test to scripts/ubuntu-container-e2e.sh to verify that default Vim startup on Ubuntu 24.04 resolves ~/.vimrc and applies number / relativenumber.

Verification

• bash scripts/check.sh (all 21 test suites, ShellCheck, shfmt, syntax checks) passed.
• bash scripts/macos-configuration-e2e.sh passed.
• Ubuntu 24.04 container E2E (scripts/ubuntu-container-e2e.sh with Vim startup regression check) passed in CI.