docs: sync against icp-cli v1.6.0, motoko-core v2.6.2, @icp-sdk/canisters 4.0.0 and static-site v0.4.0

[marc0olo]

Summary

Bumps four watched pins in .sources/upstream.json after checking each release against the docs.

• icp-cli v1.6.0: cli.md is unchanged and the release has no user-facing command changes (macOS binary signing, internal crate split). Moves the CLI docs slug 1.5 → 1.6 across docs/, AGENTS.md and .agents/upstream-tracking.md. Every linked path and anchor was checked live at /1.6/.
• motoko-core v2.6.2: .vals() is deprecated in favour of .values(); updated the two uses in guides/chain-fusion/bitcoin.mdx.
• @icp-sdk/canisters 4.0.0: only the peer requirements changed (@icp-sdk/core ^6, @dfinity/utils ^5). The AssetManager type declarations are identical to 3.6.0, so the >= 3.5.0 minimum stays correct. No page changes.
• static-site recipe v0.4.0: pins certified-assets v0.4.0 (pages already synced in chore: sync static-site docs to dfinity/certified-assets v0.4.0 #406/chore: sync static-site docs to dfinity/certified-assets 853c291 #409). Bumps @dfinity/static-site@ examples to v0.4.0, and this site's own icp.yaml (was v0.3.1). v0.4.0 now sets Path=/ on the ic_env cookie, so the Vite dev-server snippets that simulate it get Path=/ too (otherwise a nested route in dev scopes the cookie to that directory).

Closes #404, closes #411, closes #412, closes #413