Add strategy-based post selection and comment safety valve

[fumikito]

Summary

Closes #8

• Replace raw SQL post selection with strategy-based WP_Query selection (Boswell_Strategy_Selector)
• Add 4 filter hooks for external customization: boswell_comment_strategies, boswell_select_post_args, boswell_post_context, boswell_should_comment
• Enrich AI prompt with post date, categories, and "Why This Post" context section
• Add wp boswell strategies CLI command and --strategy option to wp boswell run
• Remove deprecated select_post() method and boswell_select_post filter
• Add Wiki documentation for strategy customization

Test plan

• npm test — 58 tests pass (12 new strategy tests + 3 commenter context tests)
• composer lint — No PHPCS errors
• wp boswell strategies lists registered strategies with weight/probability
• wp boswell run --persona=persona --strategy=recent generates a comment using the specified strategy
• boswell_should_comment filter returning false blocks comment on all paths (cron, CLI, MCP, REST)

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