Add create-worktree skill and .worktrees isolation

[javiercn]

Summary

Agents frequently create a git worktree to work in. Done naively in this repo, a nested worktree pollutes the main checkout's git status, fails to build (empty submodules, no SDK), or risks inheriting the top-level build configuration. This PR adds a committed, project-local worktree convention plus a Copilot skill that teaches agents how to use it.

What's in here

• .worktrees/ committed isolation (the convention):

  • .worktrees/.gitignore ignores every worktree subdirectory, so worktrees created under it never show up in the parent's git status (no per-clone .git/info/exclude needed).
  • .worktrees/.editorconfig (root = true), Directory.Build.props/.targets (empty <Project />), and NuGet.config (<clear/>) form a traversal ceiling so nothing under .worktrees/ climbs into the repo-root MSBuild/NuGet/editorconfig configuration.

• .github/skills/create-worktree/SKILL.md — a knowledge-only skill (no scripts) covering the three repo-specific things plain git worktree knowledge misses:

  1. Create the worktree under .worktrees/<name> (and copy the barrier files if it is placed elsewhere in the repo).
  2. Initialize the submodules a fresh worktree leaves empty (MessagePack-CSharp, googletest).
  3. Share the main checkout's already-provisioned .dotnet SDK via a junction/symlink when global.json sdk.version matches, instead of re-downloading gigabytes.

• eng/skill-evals/create-worktree/eval.vally.yaml + eng/skill-evals/skill-vs-baseline.experiment.yaml — a Vally A/B eval (baseline vs skill), matching the move to Vally for skill evals.

Why a worktree under .worktrees/ is isolated

A worktree is a full checkout, so its own root Directory.Build.props/.targets, global.json, and .editorconfig (root = true) terminate MSBuild/SDK/editorconfig discovery before it reaches the repo root (these are stop-at-first-match). NuGet.config is the one that merges up the tree, but the repo's <clear/> makes the worktree's own sources win. The committed .worktrees/ barrier files harden this for any loose content placed directly under .worktrees/.

Eval results (Vally, runs=5, claude-opus-4.6)

Stimulus	Baseline	With skill
Create a worktree to work on a branch	0/5	5/5
Avoid re-downloading the SDK per worktree	3/5	5/5
In-repo worktree config isolation	5/5	5/5
Build fails: missing submodule sources	5/5	5/5

The skill activates in every skilled run and wins the two action scenarios (worktree placement, SDK sharing); it is neutral where the unaided agent is already strong.

Run it from an isolated empty directory (the stimuli ask the agent to create a worktree, so it acts on the cwd):

mkdir /tmp/cw-eval && cd /tmp/cw-eval
vally experiment run <repo>/eng/skill-evals/skill-vs-baseline.experiment.yaml --output-dir .

[Copilot]

Pull request overview

This PR introduces a repository-local convention for creating isolated git worktrees under .worktrees/, and adds a Copilot skill plus Vally evals to teach and measure the correct worktree setup for this repo (submodules + SDK reuse + config isolation).

Changes:

• Add committed isolation “ceiling” files under .worktrees/ (.gitignore, .editorconfig, Directory.Build.props/.targets, NuGet.config) to prevent nested worktrees from polluting git status and from inheriting outer-repo configuration.
• Add a knowledge-only Copilot skill (.github/skills/create-worktree/SKILL.md) describing the repo-specific worktree workflow (placement, submodule init, .dotnet linking).
• Add Vally A/B evaluation specs to compare baseline vs skill-assisted behavior.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
eng/skill-evals/skill-vs-baseline.experiment.yaml	Defines the baseline vs skilled A/B experiment varying /environment/skills.
eng/skill-evals/create-worktree/eval.vally.yaml	Capability eval stimuli/rubrics for worktree placement, submodule init, SDK reuse, and isolation explanation.
.worktrees/.gitignore	Ignores all worktree directories under .worktrees/ while keeping the isolation files tracked.
.worktrees/.editorconfig	Stops EditorConfig traversal at .worktrees/ for content placed directly under it.
.worktrees/Directory.Build.props	MSBuild traversal ceiling for content placed directly under .worktrees/.
.worktrees/Directory.Build.targets	MSBuild traversal ceiling for content placed directly under .worktrees/.
.worktrees/NuGet.config	NuGet traversal ceiling intended to prevent repo-root config merging under .worktrees/.
.github/skills/create-worktree/SKILL.md	Documents the repo-specific “create an isolated worktree” procedure for agents/users.