[feat]: Update `<pf-v6-panel>` for PatternFly v6

Start with the existing elements/pf-v5-panel/ implementation, then read .claude/ADVICE.md and inspect elements/package.json, eslint.config.js, _snippets/, and framework-integration/ references. Compare the Panel requirements with the PatternFly v6 design and the listed API, demo, accessibility, and review checks. Done means the v5 element is replaced, the v6 API and visual behavior are covered, checks pass, a changeset is added, and the PR targets staging/pfv6.

Summary

Update `` to implement the PatternFly v6 Panel component.

Generic content container with optional header, footer, and scrollable body.

Prior Art

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` Panel
• MAY adjust element API to leverage web platform strengths
• SHOULD maintain CSS custom property theming compatibility
• SHOULD expose useful CSS shadow parts

Element-specific considerations

• React `variant`: `raised`, `bordered`, `scrollable` - map to `variant` attribute
• React `isScrollable` - applies overflow scroll to body; also expressible as `variant="scrollable"`
• React sub-components: `PanelHeader`, `PanelMain`, `PanelMainBody`, `PanelFooter` - use named slots
• `scrollMaxHeight` on `PanelMain` - CSS custom property preferred
• Simple structural element; complexity is in slotting composition

Checklist

Rename

• Delete `elements/pf-v5-panel/`
• Update `elements/package.json` exports
• Update `eslint.config.js` ignore globs if applicable
• Update docs `_snippets/` and `framework-integration/` references

Implementation

• API per `.claude/ADVICE.md`
• CSS uses v6 tokens, logical properties, nesting
• `@summary` and JSDoc on element class

Demos

• Demos match patternfly.org naming/structure
• CSS custom properties match computed styles from patternfly.org (Chrome MCP)
• Visual parity verified via Chrome MCP

Tests

• Public API covered (attributes, properties, events, slots)
• `a11ySnapshot` assertions

Reviews

• `/review-api` passes
• `/review-demos` passes
• `/review-a11y` passes
• `eslint` and `stylelint` pass

Ship

• Add changeset
• PR targets `staging/pfv6` (NOT `main`)