core(cache): apply %variables% substitution to goto tool + replay

[BABTUNA]

Why

keys already supports %variableName% substitution in both live execution and cache replay, but goto currently uses raw URL text in both paths.

That causes variable-driven navigation instructions to drift between intent and execution, especially when cached runs replay navigation steps.

Gap

• Live path: goto tool does not substitute %variable% tokens before page.goto(...).
• Replay path: AgentCache does not substitute %variable% tokens when replaying cached goto steps.

Proposed fix

• Add variable substitution in goto tool execution before navigation.
• Keep recorded replay step URL tokenized (do not expand variables into cache records).
• Apply variable substitution during AgentCache replay for goto steps.
• Add focused unit tests for both live tool behavior and replay behavior.

Acceptance criteria

• %variable% tokens in goto URLs are resolved before navigation.
• Cached goto steps replay with variable substitution.
• Existing goto behavior for non-variable URLs remains unchanged.