Remove experimental oracles flag - oracles are now stable

[Copilot]

Remove --experimental-oracles CLI flag and experimental warnings from oracles feature

✅ All changes completed and verified:

• Remove --experimental-oracles CLI flag from TestArgs in crates/forge/src/lib.rs
• Remove experimental warnings from documentation in docs/src/snforge-advanced-features/oracles.md
• Remove CLI flag documentation section in docs/src/appendix/snforge/test.md
• Remove experimental check in runtime extension in crates/cheatnet/src/runtime_extensions/forge_runtime_extension/mod.rs
• Remove experimental_oracles field from ForgeExtension struct and make oracles always enabled
• Remove experimental_oracles field from TestRunnerConfig in crates/forge-runner/src/forge_config.rs
• Remove experimental_oracles field from RuntimeConfig in crates/forge-runner/src/forge_config.rs
• Update combine_configs function in crates/forge/src/combine_configs.rs
• Update test code that sets experimental_oracles: false to remove the field
• Add entry to CHANGELOG.md Unreleased section only (without modifying any released versions)
• Build and test changes to ensure nothing breaks

Verification Results:

• ✅ All modified crates compile successfully
• ✅ Unit tests pass (combine_configs, forge_runner, cheatnet)
• ✅ Binary builds successfully
• ✅ CLI help output confirmed flag is removed
• ✅ No remaining references to experimental oracle flags
• ✅ CHANGELOG properly updated: new entry in Unreleased section, released versions unchanged

Summary:
The oracle feature is now stable and always enabled. Users no longer need to pass --experimental-oracles flag or set SNFORGE_EXPERIMENTAL_ORACLES environment variable. All experimental warnings have been removed from documentation.

Original prompt

Oracles are becoming stable. Remove --experimental-oracle CLI flag everywhere (including documentation) and any mentions telling that oracles are experimental.

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