[3.x] Revert generated robots.txt and llms.txt (stacked PR 6 and PR 7)

[emmadesilva]

Summary

Reverts both #2531 (generated robots.txt) and #2534 (generated llms.txt). Both were overengineered; they're being redesigned from scratch, starting from a clean baseline instead of iterating on top of the current implementation.

llms.txt is included in the same PR rather than a separate one because it was explicitly built "wired like robots.txt" — reverting only one would have left the epic and several docs full of references to a sibling feature that no longer exists.

• Removes RobotsTxtGenerator, LlmsTxtGenerator, their Features::hasRobotsTxt()/hasLlmsTxt() gates, HydeCoreExtension wiring, and the hyde.robots/hyde.llms config blocks (kept in lockstep between config/hyde.php and the framework config stub).
• Deletes the feature-specific test classes, and trims the now-dead hyde.robots.enabled/hyde.llms.enabled toggles out of collection/manifest/realtime-compiler tests that only needed them to keep exact-count assertions accurate.
• Rolls back the public docs (customization.md, build-tasks.md, in-memory-pages.md) and UPGRADE.md/HYDEPHP_V3_PLANNING.md. Illustrative uses of robots.txt as an example non-HTML filename (InMemoryPage output inference, showInSitemap()) are untouched — those describe PR 1/PR 4 behavior, not the reverted generators.
• Updates EPIC_NON_HTML_PAGES.md: PR 6 and PR 7 are marked reverted rather than implemented, their implementation notes (which described deleted code as existing) replaced with a short record of what was tried and why it was cut, and downstream references that claimed the D5 user-override paths were verified for both pages are corrected.

Test plan

• vendor/bin/pest --testsuite FeatureFramework,UnitFramework,FeatureHyde,"Realtime Compiler" — ran before and after the revert; identical FAILED output both times (only the known pre-existing FeaturedImageUnitTest PHP 8.5 failure)
• php monorepo/HydeStan/run.php — no errors
• Verified config/hyde.php and packages/framework/config/hyde.php stay in lockstep (HydeConfigFilesAreMatchingTest)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0899467) to head (d3ddfaf).

Additional details and impacted files

@@ Coverage Diff @@## v3/non-html-pages #2580 +/- ##
=====================================================
Coverage 100.00% 100.00% + Complexity 1824 1792 -32 
=====================================================
Files 184 182 -2 Lines 4539 4453 -86 =====================================================
- Hits 4539 4453 -86 

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