Redesign heading-health plugin to match Crawlith plugin standards

[saurabhsharma2u]

Motivation

• Standardize the heading-health plugin to follow the Crawlith plugin architecture, lifecycle hooks, and secure DB/config patterns described in the agent docs.
• Improve separation of concerns so analysis, scoring, CLI registration, persistence and output are easier to maintain and test.
• Provide URL-scoped caching and persistence through the ctx.db API and allow a force-refresh option for recomputations.

Description

• Restructured the package into the canonical manifest + src/ layout and exported HeadingHealthPlugin from index.ts delegating to src/ modules (plugin.ts, cli.ts, HeadingHealthService.ts, Output.ts, types.ts, analyzer.ts).
• Implemented lifecycle hooks: onInit registers schema via ctx.db.schema.define, onMetrics computes per-page payloads, reads 24h cached rows via ctx.db.data.find(..., { maxAge: '24h', global: true }), persists via ctx.db.data.save, and publishes a snapshot summary to ctx.metadata, and onReport attaches the snapshot summary to result.plugins.headingHealth.
• Extracted scoring, aggregation and grouping logic into HeadingHealthService and improved analyzer code with clearer types and utility functions (jaccardSimilarity, entropy calculation, duplicate-risk enrichment); added HeadingHealthOutput for logging and registerHeadingHealthCli for CLI options including --heading-force-refresh.
• Added/updated typed interfaces in src/types.ts, expanded JSDoc comments across modules, and updated tests to assert DB find/save calls, per-page payload attachment, and report summary behavior.

Testing

• Ran unit tests with pnpm --filter @crawlith/plugin-heading-health test and all tests passed (5 passed (5)), validating analyzer behavior and plugin hooks. ✅
• Attempted package build with pnpm --filter @crawlith/plugin-heading-health build but the isolated TypeScript compile failed because the environment could not resolve the workspace package @crawlith/core during an isolated package tsc run; this is an environment workspace resolution issue and does not affect the plugin unit tests. ⚠️

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Codex Task