feat: add Windows PowerShell support and remote install one-liners

- remote-install.sh: POSIX one-liner (curl | sh), downloads archive to tmpdir
- remote-install.ps1: Windows one-liner (irm | iex), fully native PowerShell
- install.ps1: Native Windows installer calling all .ps1 post-install scripts
- bin/mosaic-link-runtime-assets.ps1: Syncs runtime config files
- bin/mosaic-sync-skills.ps1: Clones skills, links via directory junctions
- bin/mosaic-migrate-local-skills.ps1: Migrates local skills to junctions
- bin/mosaic-doctor.ps1: Health audit for Windows environments

Directory junctions used instead of symlinks (no elevation required).
All junction operations fall back to file copy on failure.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

remote-install.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env sh
+# Mosaic Bootstrap — Remote Installer (POSIX)
+#
+# One-liner:
+#   curl -sL https://git.mosaicstack.dev/mosaic/bootstrap/raw/branch/main/remote-install.sh | sh
+#
+# Or with wget:
+#   wget -qO- https://git.mosaicstack.dev/mosaic/bootstrap/raw/branch/main/remote-install.sh | sh
+#
+set -eu
+
+ARCHIVE_URL="https://git.mosaicstack.dev/mosaic/bootstrap/archive/main.tar.gz"
+TMPDIR_BASE="${TMPDIR:-/tmp}"
+WORK_DIR="$TMPDIR_BASE/mosaic-bootstrap-$$"
+
+cleanup() {
+  rm -rf "$WORK_DIR"
+}
+trap cleanup EXIT
+
+echo "[mosaic] Downloading bootstrap archive..."
+
+mkdir -p "$WORK_DIR"
+
+if command -v curl >/dev/null 2>&1; then
+  curl -sL "$ARCHIVE_URL" | tar xz -C "$WORK_DIR"
+elif command -v wget >/dev/null 2>&1; then
+  wget -qO- "$ARCHIVE_URL" | tar xz -C "$WORK_DIR"
+else
+  echo "[mosaic] ERROR: curl or wget required" >&2
+  exit 1
+fi
+
+if [ ! -f "$WORK_DIR/bootstrap/install.sh" ]; then
+  echo "[mosaic] ERROR: install.sh not found in archive" >&2
+  exit 1
+fi
+
+echo "[mosaic] Running install..."
+cd "$WORK_DIR/bootstrap"
+bash install.sh
+
+echo "[mosaic] Cleaning up temporary files..."
+# cleanup runs via trap