akm v0.5.0 migration notes

• New top-level surfaces: akm wiki …, akm workflow …, akm vault …, and akm save.
• If you tried the unreleased single-wiki LLM prototype, move to the new akm wiki … workflow.
• Removed from the prototype surface: akm lint, akm import --llm, akm import --dry-run, knowledge.pageKinds, and the old ingest/lint LLM prompts.
• Existing raw wiki-like content should be moved into wikis/<name>/raw/ and then managed with the new wiki commands.