feat: Landing polish: README fixes, contributor docs cleanup, mode-aware playground Reset

[pranshuparmar]

Description

Follow-up polish after the top-section refresh landed on main:

• README — the hero banner gets a descriptive alt text (was the filename witr_banner_final) and an explicit width="1232"; the tagline now reads "process, port, container, or file", matching the About description and witr's actual --container capability.
• Contributor docs — the lone note in .agent/workflows/validate-pr-check.md moved into CONTRIBUTING.md as a "Validating the PR Check Locally" subsection under the Pull Request Process, and the .agent/ directory is removed from the repo root.
• Playground — Reset now respects the current mode. Previously, clicking Reset from free play relaunched the tutorial cold open, because resetScenario() unconditionally re-entered the scenario. World restoration is now split from tutorial entry: mid-tutorial Reset restarts the incident; free-play Reset hands back a pristine box while staying in free play. Scenario switches preserve the mode the same way. Verified end-to-end in a browser (mode preserved, world restored after kills, Tutorial button unaffected) and the fidelity check still passes 16/16.

The banner image itself will be swapped in the README separately (uploaded manually as a repo attachment, not versioned in-repo).

Type of change

Check all that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (may affect existing functionality)
• This change requires a documentation update

Checklist

• I have formatted my code using go fmt ./... _{(no Go source changed)}
• I have opened this PR against the staging branch _{(targets main intentionally — README + playground, and Pages deploys from main /docs)}
• I have performed a self-review of my own code
• I have added helpful comments where needed
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes _{(playground fidelity check: 16/16; browser test of the Reset flow: 7/7)}

Thank you for your contribution! 🎉

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