chore(repo): drop orphan dist/ artifact and gitignore the directory

dist/PowerShell.MCP.psm1 was the only git-tracked file under dist/.
It was unreferenced by any code path: Build-AllPlatforms.ps1 writes
to the user''s PSModulePath, .github/workflows/release.yml stages
from Staging/, and grep finds zero references to dist/* anywhere
in the codebase. The file was a stale snapshot from an older
local-distribution workflow that no longer exists.

The other dist/ contents (release/, bin/, mcpb-stage/, skills/) were
already excluded from git via the [Rr]elease/ rule incidentally
matching dist/release/ — but that''s coincidence, not intent. Add
an explicit dist/ rule so future build-output cruft can''t sneak
back in.

Side benefit: the comprehensive review surfaced "three-way drift"
between dist/, dist/release/, and Staging/ psm1 files (different
versions, different pipe-name formats, an Install-ClaudeSkill
function only present in the oldest snapshot). All of that was
local-disk rot — the agent was inspecting .gitignore-d cruft. With
dist/ explicitly ignored and the orphan file removed, the apparent
"three-way drift" simply disappears: Staging/ is the only source.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yotsuda

.gitignore

@@ -21,6 +21,9 @@ mono_crash.*
 [Dd]ebugPublic/
 [Rr]elease/
 [Rr]eleases/
+# Local build artifact directory — not consumed by anything anymore
+# (Staging/ is the source of truth for releases via .github/workflows/release.yml).
+dist/
 x64/
 x86/
 [Ww][Ii][Nn]32/