Update worktree instructions

[anod]

Summary

• document creating task worktrees under the ignored wt/ folder
• document copying ignored local build inputs into task worktrees before building
• ignore the local wt/ workspace folder

[Copilot]

Pull request overview

This PR updates repository workflow documentation to standardize where task git worktrees are created and how to handle ignored local build inputs needed for Android builds.

Changes:

• Document creating task worktrees under an ignored wt/ folder and cleaning them up after merge.
• Document copying ignored/local build inputs (e.g., local.properties, app/google-services.json) into task worktrees before building.
• Add /wt/ to .gitignore to keep local worktree directories untracked.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

File	Description
.gitignore	Ignores the root-level wt/ folder intended to hold task worktrees.
.github/copilot-instructions.md	Updates worktree/build guidance to use wt/ and to copy required ignored build inputs into worktrees.