fix(plugin): sync adoption Scope section into claude-plugin + track its .mcp.json

Rebasing feat/adoption-contract-injection onto the claude-plugin bundle
surfaced two byte-identity/build gaps in the bundle base:

- claude-plugin/skills/using-tracedecay/SKILL.md lacked the adoption
  "## Scope and priority" section (SUBAGENT-STOP guard + user-instruction
  priority ladder) that the codex/cursor copies carry. Copied the section
  in so all three using-tracedecay/SKILL.md copies are byte-identical, as
  the cross-surface byte-identity tests (claude_bundle_skills_stay_byte_
  identical_to_the_codex_source, plugin_bundle_sync) require.

- claude-plugin/.mcp.json was referenced by src/agents/claude.rs via
  include_str! but never committed: .gitignore ignores `.mcp.json` and only
  negated codex-plugin/.mcp.json. The lib failed to build. Added the file
  (byte-identical to codex-plugin/.mcp.json: stdio serve + global-DB env)
  and a matching `!claude-plugin/.mcp.json` gitignore negation so it is
  tracked. Satisfies claude_bundle_mcp_config_declares_the_tracedecay_server.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: ScriptedAlchemy

claude-plugin/skills/using-tracedecay/SKILL.md

@@ -12,6 +12,16 @@ code, reading code, tracing calls, estimating blast radius, recalling prior
 context — try the matching tracedecay tool BEFORE Grep, Glob, codebase
 search, or file reads. You cannot rationalize your way out of this.
 
+## Scope and priority
+
+- SUBAGENT-STOP: if you are a scoped subagent that was handed the exact files,
+  symbols, or excerpts to act on, do NOT re-run discovery — act on what you
+  were given. This mandate governs open-ended exploration, not narrow handoffs.
+- Priority ladder: explicit user instructions and project rules
+  (CLAUDE.md / AGENTS.md) win over this skill; this skill wins over the host's
+  default "just grep it" habit. Never fight a direct instruction to satisfy
+  the mandate.
+
 ## Moment → mandatory action
 
 | The moment you are in | Do this first |