docs(routing): LKGP sticks at provider level — align copy with routerStrategy contract (#11555)

Validated in a combined 4-PR batch worktree off release/v3.8.51 tip. Also removed a leftover changelog placeholder duplicate (0000-lkgp-doc-provider-stickiness.md, byte-identical content to the correctly-numbered 11555- fragment minus the PR link) — pushed to this branch.
- Focused test: router-strategies.test.ts — part of batch's 60/60 node:test run
- typecheck:core, file-size, changelog-integrity, complexity, cognitive-complexity, check:docs-counts-sync — all OK
- Full-repo lint: 228 pre-existing dashboard react-hooks/* findings, unrelated to this diff

Thanks for the docs-accuracy follow-through — aligning the LKGP copy with what routerStrategy.ts actually does (provider-level, not target/model) closes a real gap between prose and behavior.

docs/routing/AUTO-COMBO.md

@@ -280,7 +280,7 @@ OmniRoute's combo engine supports **19 routing strategies** (declared in `src/sh
 | `headroom`          | Pick the target with the most remaining quota headroom                                                                                                                                    |
 | `strict-random`     | Random without deduplication of repeats                                                                                                                                                   |
 | `auto`              | Use Auto Combo scoring (15-factor) — **recommended**                                                                                                                                      |
-| `lkgp`              | Last-Known-Good Path (sticky route to last successful target)                                                                                                                             |
+| `lkgp`              | Last-Known-Good Path (pins to the last successful provider, then falls back to rules)                                                                                                     |
 | `context-optimized` | Pick target with best fit for current context size                                                                                                                                        |
 | `cache-optimized`   | Reorder targets by prompt-cache affinity — the connection likeliest to already hold this request's cached prefix is tried first (`open-sse/services/combo/promptCacheAffinity.ts`, #8008) |
 | `fusion` 🧬         | Fan out to a panel of models in parallel, then synthesize one answer via a judge (see below)                                                                                              |