docs: add concurrency guidance and lock contention warnings to skills

[stack72]

Summary

• Adds a design checkpoint to the swamp-model skill's Workflow Example that prompts users to consider concurrency before creating a model — asking whether methods will run concurrently or be long-running, and guiding them to the factory pattern
• Updates the factory pattern description to call out both data reuse AND concurrency as motivations (separate instances = separate locks)
• Adds a lock contention warning to the workflow data-chaining reference, clarifying that steps on the same model serialize on the per-model lock despite being in the same job

Addresses swamp.club/lab#43 — users hitting model locks during long-running operations because they didn't know to split concerns into separate model instances.

Test plan

• deno fmt --check passes
• deno lint passes
• deno run test passes (4280 tests)
• Verify skill triggers the design checkpoint when a user asks to create a model with multiple methods

🤖 Generated with Claude Code

[github-actions]

Code Review

Blocking Issues

None.

Suggestions

1. SKILL.md exceeds 500-line limit — .claude/skills/swamp-model/SKILL.md is already 539 lines on main, and this PR adds ~18 net lines (bringing it to ~557). CLAUDE.md requires keeping SKILL.md under 500 lines, splitting detailed content into references/ files. The new "Design check" section (16 lines) is a good candidate to move into a reference file (e.g., references/concurrency-design.md) with a one-line pointer from SKILL.md. This is a pre-existing violation so it shouldn't block this PR, but worth addressing in a follow-up.

Notes

• Content is technically accurate: the per-model exclusive lock behavior and factory pattern guidance for parallelism are correct and address a real user pain point (swamp.club/lab#43).
• The lock contention warning in data-chaining.md is a valuable addition — the previous text ("Steps within a job run in parallel") was misleading without the serialization caveat.
• No code changes, so no test/type-check/lint concerns.
• No security issues.
• No libswamp import boundary concerns (documentation-only change).