Fork agent SCI state with Room worlds

[whilo]

Summary

• compose each long-lived agent SCI interpreter as a forkable Spindel world component
• select fork-local interpreter state, Room capabilities, environment, filesystem/Git, and Datahike connections through the active execution context
• preserve retained pre-fork functions and FRP constructors while rebinding them to the child Room incarnation
• isolate and dispose process-global scratch Datahike databases per interpreter world
• make Room/context creation, hydration, publication, and unregister cleanup failure-atomic
• extend the provider-free REPL harness to prove fork-local SCI vars and Datahike writes diverge from the parent

Lifecycle guarantees

• one concurrent first allocation produces one ChatContext and one SCI component
• fallible durable hydration completes before any ChatContext signal/component allocation
• namespace-binding failure releases the newly registered component
• Room unregister reservations close admission before process-local teardown
• inbound attention arriving during hydration waits for complete cache publication
• disposable-resource cleanup failure cannot skip SCI component unregister
• fork registration fences run before atomic Room/topology publication

Verification

• clojure -M:format
• clojure -M:test --seed 2104642063 without provider credentials: 634 tests, 2,803 assertions, 0 failures
• standalone harness build and all CircleCI jobs green
• independent exact-head review of 3b25abc: no medium/high findings

Dependency order

This PR is stacked on #85 and temporarily pins the reviewed dependency commits so CI exercises the real combined stack:

1. SCI forkable worlds: Add forkable SCI runtime worlds babashka/sci#1084; release candidate Release SCI 0.15.59 whilo/sci#2 (0.15.59)
2. Spindel world components: feat: compose SCI as a forkable world component spindel#48
3. this PR

After SCI and Spindel are released, replace both Git pins with their Maven coordinates before releasing Dvergr.