Install treemon-cli skill to ~/.copilot and update SKILL.md guidance

[0101]

Problem

Two related gaps in how the treemon-cli skill is shipped:

1. Install-Skill in treemon.ps1 only installed SKILL.md to ~/.claude/skills/treemon-cli/. The GitHub Copilot CLI uses the same skill format under ~/.copilot/skills/, but installs there had to be done by hand.
2. A manual edit to ~/.copilot/skills/treemon-cli/SKILL.md (adding a "Default workflow when this skill is explicitly invoked" section) had drifted from the repo source of truth at src/Cli/skill/SKILL.md.

Changes

• treemon.ps1 / Install-Skill — Add a second install block for ~/.copilot/skills/treemon-cli/, mirroring the existing .claude block. Same shape, same Join-Path style, same $installed array (now also collects "GitHub Copilot CLI"). The "no supported AI tool directories found" warning now lists both ~/.claude/skills/ and ~/.copilot/skills/ as missing instead of only Claude.
• src/Cli/skill/SKILL.md — Port the pending manual edit into the repo source:
  • Add a new ## Default workflow when this skill is explicitly invoked section describing the explicit-invocation workflow (pick branch name → write prompt file → tm new → tm launch → report back).
  • Add a ### When the user did NOT invoke this skill explicitly subsection clarifying auto-load behaviour.
  • Tweak the tm worktrees description to mention looking up a freshly created worktree's path.
  • Expand the "When to use" bullets so the explicit-invocation case is the first entry.

treemon.ps1 deploy already calls Install-Skill, so deploys now pick up the Copilot destination automatically.

Tests

treemon.ps1 install-skill reports installs for both clients, and Get-FileHash on all three locations matches:

4122E7EDD3C9D3B126F23038970ED3DCD8BCB21B13F23368A57A0545921B11DA  src/Cli/skill/SKILL.md
4122E7EDD3C9D3B126F23038970ED3DCD8BCB21B13F23368A57A0545921B11DA  ~/.claude/skills/treemon-cli/SKILL.md
4122E7EDD3C9D3B126F23038970ED3DCD8BCB21B13F23368A57A0545921B11DA  ~/.copilot/skills/treemon-cli/SKILL.md

No F# changes, so no dotnet build / dotnet test runs were required.