sdk: nothing tells a plugin author that Task.Fn runs concurrently — the SDK synchronizes itself and never says the author must too

Problem

Concurrent invocations of one plugin task's Fn are a normal occurrence, not an edge: the host's transport allows up to 8 connections per plugin (pkg/flowstate/v1/plugin/transport.go:95, MaxConnsPerHost: 8), and the SDK serves each Connect request on its own goroutine with no serialization around task.Fn. Two steps calling the same task — in one run's parallel branches or across concurrent runs on one worker — execute Fn concurrently.

The SDK knows this about itself: its progress-reporting path takes a mutex precisely because the underlying stream "is not safe for concurrent use" (sdk/sdk.go:1062-1065). It never passes the requirement on: neither sdk.Task/TaskFunc's doc comments nor docs/PLUGINS.md state that Fn must be safe for concurrent use. PLUGINS.md's "Five places the contract is implicit" and "Three traps the code knows about and no authoring surface teaches" sections — which exist to enumerate exactly this class of unwritten contract — do not mention it. An author who keeps unsynchronized package-level state (a lazily-built client, a counter, a cache) gets no signal until it breaks under load, where it is least debuggable.

Desired outcome

State the contract where authors read: sdk.Task.Fn/TaskFunc doc comments say "safe for concurrent use," and PLUGINS.md's implicit-contract section gains the entry, including the one nuance worth a sentence — per-call state is fine, shared mutable state needs the same care any Go HTTP handler needs. If the conformance direction (#713's harness) grows teeth, a concurrent-invocation probe is a natural black-box check.

Acceptance

• The doc comment and PLUGINS.md both state the requirement; the walkthrough's own example remains a correct model (it already is — pure per-call code).
• Whether the connection ceiling should be operator-tunable is explicitly out of scope here (adjacent to #1309's bounding discussion), so this stays a documentation-contract fix.

Adjacency

#713 (the conformance harness this could feed), #1309 (operator ceilings on plugin RPC — the tunability half lives there if anywhere), PLUGINS.md's implicit-contract sections (the pattern this completes).

Start with the sdk.Task.Fn and TaskFunc doc comments, then read the implicit-contract sections in docs/PLUGINS.md. Document that Fn must be safe for concurrent use, including the distinction between per-call state and shared mutable state. Confirm that the walkthrough example remains correct and that both documented locations state the requirement.