Add installable agent skill (itrx-skill)

[virgesmith]

Equivalent of virgesmith/xenoform-rs#24 for itrx.

Why

Coding agents write correct Itr chains more reliably when given a compact, purpose-built reference than when relying on the whole README.md being in context. Bundling one that agents can install makes that reference available in any downstream project using itrx, not just this repo.

What

• src/itrx/skill/SKILL.md — the bundled skill. Covers construction, the lazy/eager split (including which non-lazy methods short-circuit and so are still safe on infinite sources), collect targets, when reaching for Itr is actually worth it, and the pitfalls: single-pass/no-rewind, tee/copy shared-buffer growth, groupby/sorted_by sorting up front vs lazy chunk_by, eager value_counts/rev, dedup being adjacent-only, 0-based nth, next/last raising where find returns None, product materialising other.
• src/itrx/skill_cli.py + [project.scripts] — an itrx-skill console script with --install [PATH] / --remove [PATH] (default PATH=.agents), creating PATH/skills/itrx.
• src/test/test_skill_cli.py — full coverage of both subcommands.
• README.md — an "Agent skill" section plus a pointer from the install section; relnotes.md — an ## Unreleased entry.

Design decisions

• Symlink into the target project rather than copy — the link points at the skill/ directory inside whatever itrx is installed in the active environment, so the skill always matches the installed version with nothing to go stale. Modelled on Streamlit's streamlit skills CLI, which takes the same approach for the same reason.
• Bundled inside the package rather than fetched from a URL — no network dependency for install, and verified with unzip -l on a built wheel that hatchling packages src/itrx/skill/SKILL.md and registers the entry point with no extra config, same as the existing py.typed marker. (Note this differs from xenoform-rs, which uses uv_build.)
• Symlink-or-refuse, no overwriting — a refusal never touches a pre-existing real file or directory, or a symlink this script doesn't own. No multi-harness detection, no global-install mode, no prompts.
• Relative link target where one exists, absolute where none does — a relative target survives the project directory being moved, but on Windows there is no relative path between different drives and os.path.relpath raises ValueError (not OSError); that is the normal end-user layout of a venv on one drive and a project on another, and it broke every Windows job in the upstream PR. _link_target falls back to absolute there, with a test that forces the ValueError path.
• No version bump — v0.4.0 is already tagged, so the entry sits under ## Unreleased for a later release commit.

Verification

• ruff check, ruff format --check, ty check src clean; pytest 193 passed, coverage 100%.
• Console script smoke-tested end to end in a throwaway project: install → idempotent re-install → remove.

Follow-ups

Only targets PATH/skills/itrx (default .agents); multi-harness target dirs (e.g. .claude/skills) could be added later if wanted.

🤖 Generated with Claude Code