fix(taskctl): coerce numeric depends_on to strings in Composer schema (#250)

[randomm]

Fixes #250

Changes

• composer.ts: coerce numeric depends_on values to strings via zod union+transform+pipe
• agent.ts: add example task with string depends_on reference and Rule 9 clarifying no numeric indexes
• composer.test.ts: add test verifying numeric coercion works without validation errors

Root cause

Composer LLM was outputting numeric sequential IDs (e.g. [1, 2]) in depends_on arrays. The schema required strings, causing validation failure on every taskctl start.