refactor(context): deduplicate system prompt — markdown skills index, skip template MEMORY.md

- Convert skills summary from verbose XML (4-5 lines/skill) to compact
  markdown list (1 line/skill) with inline path for read_file lookup
- Exclude always-loaded skills (e.g. memory) from the skills index to
  avoid duplicating content already in the Active Skills section
- Skip injecting the Memory section when MEMORY.md still matches the
  bundled template (i.e. Dream hasn't populated it yet)

nanobot/templates/agent/skills_section.md

@@ -1,6 +1,6 @@
 # Skills
 
 The following skills extend your capabilities. To use a skill, read its SKILL.md file using the read_file tool.
-Skills with available="false" need dependencies installed first - you can try installing them with apt/brew.
+Unavailable skills need dependencies installed first — you can try installing them with apt/brew.
 
 {{ skills_summary }}