super-swarm-spark: make concurrency limit docs consistently 15

[iqdoctor]

Summary

• make super-swarm-spark concurrency documentation consistent at 15 everywhere
• update headings, scheduler bullets, and execution summary max workers to 15
• add explicit runtime config note for Codex thread limits:
  • ~/.codex/config.toml
  • [agents]
  • max_threads = 16

Why

The skill previously mixed 15 and 12 limits in different sections, which created ambiguity in expected behavior and operator setup.

Scope

Single file docs change: skills/super-swarm-spark/SKILL.md. No execution logic changes.

[xkonjin]

Quick review pass:

• This looks docs-only from the diff, so I don’t see a code-path blocker here.
• Please still verify the examples/commands in SKILL.md against current behavior so we don’t merge stale instructions.
• If there’s a linked feature change behind this, CI should include at least one smoke check that matches the documented flow.