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

[bennypowers]

Summary

Update `` to implement the PatternFly v6 Panel component.

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

Prior Art

Source	Element	Status
PFE v4	``	Exists
cem	--	--
cockpit	--	--
chickadee	--	--

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`)