Add Advisory Strategy and Smart Plan, Cheap Execution patterns

[devill]

Two complementary patterns for splitting work between an expensive and a cheap model. They read as a pair and cross-reference each other, so they go in together.

Advisory Strategy — the cheap model runs by default and consults a stronger model as an advisor on defined triggers (two failed attempts, self-contradiction, declared uncertainty). The advisor writes no code and runs no tools, so its share of the bill stays small.

Smart Plan, Cheap Execution — the strong model plans, the plan is checkpointed to a document, a cheap model executes it step by step.

Advisory Strategy is named after Anthropic's advisor strategy, which measured it: Sonnet 4.6 with an Opus advisor gained 2.7 points on SWE-bench Multilingual over Sonnet alone and cost 11.9% less per task; Haiku 4.5 more than doubled its BrowseComp score, 19.7% to 41.2%. The write-up presents the advisor arrangement as an inversion of plan-then-execute, so Smart Plan, Cheap Execution cites it as contrast rather than as evidence.

Replaces #43 and #44, which held the same two patterns on separate branches with an earlier draft of the text.

Checks

npm run validate, npm run lint, 336 unit tests, and npm run build all pass. Both pages checked in the browser: headings, the Anthropic link, and the Related sidebar (similar-to / solves Sunk Cost) render correctly.

[devill]

Superseded by #56, which renames the pattern to Advisor Strategy to match Anthropic's naming.