feat: import shared indico agent guidance

[moliholy]

This PR imports shared agent guidance from unconventionaldotdev/indico-agents as a submodule under agents/indico, and exposes its universal markdown files at host-native paths via relative symlinks.

Overview

The submodule ships:

• AGENTS.md and CODING_GUIDELINES.md: shared baseline for any Indico-based host repository.
• indico/AGENTS.md: guidance for editing files inside the upstream Indico submodule.
• skills/: reusable agent skills (locate, RHs, migrations, tests, submodule bumps) consumable by any AI assistant that reads SKILL.md files.
• scripts/install-links.sh: bootstrap that materialises symlinks idempotently.

indicorp/
├── AGENTS.md             -> agents/indico/AGENTS.md             (committed)
├── CODING_GUIDELINES.md  -> agents/indico/CODING_GUIDELINES.md  (committed)
├── indico/
│   └── AGENTS.md         -> ../agents/indico/indico/AGENTS.md   (per-clone, gitignored upstream)
├── .claude/skills/...    -> ../../agents/indico/skills/...      (per-contributor, gitignored)
└── agents/indico/        (submodule)

Universal symlinks are committed once. Skill symlinks are per-contributor: each teammate may use a different assistant (Claude Code, Codex, Cursor), so the chosen */skills/ directory is gitignored.

How to test

After cloning or pulling:

git submodule update --init --recursive

# Universal markdown (AGENTS.md, CODING_GUIDELINES.md, indico/AGENTS.md)
bash agents/indico/scripts/install-links.sh

# Plus skills for your AI assistant of choice
bash agents/indico/scripts/install-links.sh .claude/skills    # Claude Code
bash agents/indico/scripts/install-links.sh .codex/skills     # OpenAI Codex
bash agents/indico/scripts/install-links.sh .cursor/skills    # Cursor

Verify:

• readlink AGENTS.md resolves to agents/indico/AGENTS.md.
• readlink indico/AGENTS.md resolves to ../agents/indico/indico/AGENTS.md.
• git -C indico status is clean (the host-side symlink is added to the upstream submodule's local .git/info/exclude).
• ls .claude/skills/ shows each shared skill as a directory symlink, and they do not appear in git status (covered by .gitignore).

See agents/indico/HOST_INTEGRATION.md for the full integration model.