Tag filter: add tag CSS classes to path-tree items and method badges

[t1]

Part of #10 — see design spec in docs/superpowers/specs/2026-04-12-path-tree-tag-filter-design.md.

Scope: Java generator changes only.

• Add tag-{sanitized} CSS classes to tree items based on their operations' tags.
• Add tag-{sanitized} CSS classes to method badges.
• Tag name sanitization: lowercase, replace non-alphanumeric with hyphens, collapse/trim.
• Generate per-tag CSS filter rules in openapi-ui.css.
• Generate filter icon (fa-filter) and pre-rendered pill panel in path-tree HTML header.
• Only render filter UI when API has 2+ unique tags.
• Unit tests for generated HTML structure and CSS rules.

[t1]

Agent: starting work

Type: Feature
Summary: Add CSS-based tag filtering infrastructure to the path-tree view. This is the Java generator portion that emits tag classes on tree items and method badges, generates per-tag CSS filter rules, and renders the filter icon + pill panel HTML when the spec has 2+ tags.
Design decisions:

• Tag sanitization: lowercase, replace non-alphanumeric with hyphens, collapse/trim
• CSS classes: tag-{sanitized} on tree items and badges
• CSS rules: per-tag :not(.tag-{tag}):not(:has(.tag-{tag})) for tree items, :not(.tag-{tag}) for badges
• Filter UI only rendered when 2+ tags exist
  Acceptance criteria:
• Tree items get tag-{sanitized} classes based on their operations' tags
• Method badges get tag-{sanitized} classes
• openapi-ui.css contains per-tag filter rules
• Filter icon and pill panel rendered in path-tree header when 2+ tags
• Unit tests verify HTML structure and CSS generation
  Files: core/src/main/java/com/github/t1/openapiui/OpenApiUiGenerator.java, test files, demo app verification

[t1]

Agent: done

Commit: 592aef5
Summary: Added CSS-based tag filtering infrastructure to the path-tree view. Java generator now emits tag-{sanitized} classes on tree items and method badges, generates per-tag CSS filter rules for tree items and badges, and renders filter icon + pill panel HTML when the spec has 2+ tags.
Tests: Added tests for tag class generation, multiple tag classes, untagged operations, method badge classes, CSS filter rule generation, filter icon rendering, and pill panel generation
CI: passed