Document concurrency expectations for Toolset and Skill execution

[jonwraymond]

Problem

Concurrency contracts are implicit. Toolset is marked concurrency-safe, but Skill planning/execution is sequential and does not clarify thread-safety expectations for inputs.

Evidence

Files:

• toolcompose/set/doc.go notes Toolset concurrency safety
• toolcompose/skill/execute.go executes sequentially without goroutines

Proposal

Add explicit docs for concurrency guarantees and input immutability expectations for:

• Toolset
• Skill planner
• Skill executor

Acceptance Criteria

• Docs mention concurrency guarantees
• Note that input steps and tools should not be mutated concurrently

[blocksorg]

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