Add cross-agent install.sh and update install docs

[pditommaso]

Summary

• Adds install.sh for installing the SKILL.md bundles into 7 non-Claude agents (Codex, Cursor, Gemini CLI, GitHub Copilot, Goose, Windsurf, OpenCode). Per-agent destination paths verified against each agent's own docs; sources cited inline in the script header.
• Keeps the native Claude Code plugin marketplace as the primary install path (auto-updating); positions install.sh as a parallel path for everyone else.
• Updates README.md with a per-agent destination table, usage examples (--list, --symlink, --dry-run, --target, --force), and a Credits section pointing at alirezarezvani/claude-skills, whose CLI shape inspired the script.

Test plan

• ./install.sh --list renders an aligned table of all 8 agents and their default paths.
• ./install.sh --agent <name> --dry-run for all 8 agents (claude, codex, copilot, cursor, gemini, goose, opencode, windsurf) reports the correct destination and scope, plus per-agent post-install hints where applicable.
• ./install.sh --agent codex --target <tmp> actually copies all 4 skills.
• Re-running without --force in a non-TTY context skips with a clear warning (no overwrite).
• --force re-run overwrites cleanly.
• --symlink creates absolute symlinks back into the repo (so git pull propagates updates).
• --agent bogus and missing --agent exit with code 2 and a clear error.

🤖 Generated with Claude Code