feat: validate slug format in axme_save_decision and axme_save_memory

[George-iam]

When saving decisions or memories, the slug parameter is used as part of the filename (e.g., D-042-my-decision-slug.md). Currently there is no validation that the slug conforms to a safe kebab-case format. Slugs with spaces, special characters, or uppercase letters could cause filesystem issues or dedup failures. Add a simple validation that rejects slugs not matching /^[a-z0-9][a-z0-9-]*$/ and returns a helpful error message.

Relevant files:

• src/tools/decision-tools.ts — add slug validation in axme_save_decision handler
• src/tools/memory-tools.ts — add slug validation in axme_save_memory handler
• test/decisions.test.ts and test/memory.test.ts — add test cases for invalid slugs

Acceptance criteria:

• Slugs not matching ^[a-z0-9][a-z0-9-]*$ are rejected with a clear error message
• Error message suggests the correct format (lowercase, hyphens, no spaces)
• At least 4 test cases: valid slug, slug with spaces, slug with uppercase, slug with special chars
• npm test passes