skeleton: plugin layout + docs, skill/agent migration deferred to iter 1

Establishes the repository structure for the skills plugin:
- README + INSTALL describing the two-layer split (plugin
  mechanics vs per-project profile)
- docs/design, profile-schema, pipeline, agent-template covering
  the universal discipline constants and the profile slot model
- templates/project-profile.yml as a copy-and-fill starting point
- templates/CLAUDE.md.fragment with the baseline orchestrator
  rules a project can import
- install.sh / uninstall.sh wiring skills/ + agents/ into
  ~/.claude/ via idempotent symlinks
- skills/ and agents/ directories empty except for migration
  READMEs; the actual SKILL bodies and agent files migrate
  from ~/dev/ailang/skills/ in the next iteration.

No skill or agent runs yet — this commit only stands up the
structure and documents the substitution model.

install.sh

@@ -0,0 +1,59 @@
+#!/usr/bin/env bash
+# Install the skills plugin globally into ~/.claude/.
+#
+# Creates symlinks from ~/.claude/skills/<name> and
+# ~/.claude/agents/<name> into this repo's skills/ and agents/
+# subdirectories. Idempotent — existing symlinks pointing to
+# this repo are left alone; existing entries that point elsewhere
+# are reported and skipped.
+
+set -euo pipefail
+
+REPO_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+CLAUDE_SKILLS="$HOME/.claude/skills"
+CLAUDE_AGENTS="$HOME/.claude/agents"
+
+mkdir -p "$CLAUDE_SKILLS" "$CLAUDE_AGENTS"
+
+link_one() {
+    local src="$1"
+    local dst="$2"
+    if [ -L "$dst" ]; then
+        local existing
+        existing="$(readlink "$dst")"
+        if [ "$existing" = "$src" ]; then
+            echo "ok    $dst -> $src"
+            return 0
+        fi
+        echo "skip  $dst already symlinked to $existing"
+        return 0
+    fi
+    if [ -e "$dst" ]; then
+        echo "skip  $dst exists and is not a symlink"
+        return 0
+    fi
+    ln -s "$src" "$dst"
+    echo "link  $dst -> $src"
+}
+
+if [ -d "$REPO_DIR/skills" ]; then
+    for s in "$REPO_DIR"/skills/*/; do
+        [ -d "$s" ] || continue
+        name="$(basename "$s")"
+        link_one "$s" "$CLAUDE_SKILLS/$name"
+    done
+fi
+
+if [ -d "$REPO_DIR/agents" ]; then
+    for a in "$REPO_DIR"/agents/*/; do
+        [ -d "$a" ] || continue
+        name="$(basename "$a")"
+        link_one "$a" "$CLAUDE_AGENTS/$name"
+    done
+fi
+
+echo
+echo "Install complete."
+echo "Next: drop a profile into each project that should use the plugin:"
+echo "  cp $REPO_DIR/templates/project-profile.yml <project>/.claude/dev-cycle-profile.yml"