docs: complete feature-flag reference for fork operators

[DerrickF]

Summary

A fork operator testing the stack in their own AWS reported the feature-flag docs were incomplete — no full list of flags, no on/off defaults, and no indication of which ones incur recurring cost. This closes that gap.

• New page configuration/feature-flags.md — every flag (~35) across both layers (CDK build-time CDK_* vars and runtime-Python readers), organized so an operator can answer what's on, what's off, what costs money in one read.
  • A deployed-default column (not the code default — see below).
  • A "Flags that cost money" section: Managed KB (~$5/GB-mo), feedback evals, MCP token enrichment (forces a paid Cognito plan), KB sync, and TOOL_SUMMARIES_ENABLED (default-on, Nova Micro per tool batch).
  • A trap callout: several flags whose Python code default is OFF are forced ON by CDK at deploy (Memory Spaces, Fine-Tuning), so reading the code default misleads you.
  • Documents the ~20 runtime-only flags that have no CDK_* variable at all — all ON, only flippable by hand-editing the task definition (previously undocumented).
• Rewrote environment-variables.md — it was a "Draft" placeholder naming 3 dead flags. Now a config-chain overview pointing at the new flags page and sibling config pages.

The Configuration sidebar auto-generates from the folder, so no nav change is needed.

Testing

• Frontmatter validated; internal links use the site's /agentcore-public-stack/ base and all four sibling targets (cors, authentication, agentcore-services, local-development) exist.
• Not run: the full Astro build (host memory constrained). Reviewer or CI should confirm the site builds and links render.