Prevent concurrent mutation of tools stored in Toolset

[jonwraymond]

Problem

Toolset is RWMutex-protected, but stores pointers to CanonicalTool. If callers mutate a tool after adding it, concurrent readers can race.

Evidence

File: toolcompose/set/toolset.go

• Add stores *adapter.CanonicalTool directly.
• Comments indicate toolset is concurrency-safe, but tool objects remain mutable externally.

Proposal

1. Deep copy CanonicalTool on Add/Update, or
2. Document immutability requirement clearly and enforce via cloning helpers.

Acceptance Criteria

• Toolset enforces immutability (copy or explicit guard)
• Concurrency tests cover mutation attempts

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