fix: export hooks subpaths and harden X-Glean env parsing

[chris-freeman-glean]

What

Follow-up to #96 to fix a couple of post-merge issues:

• Export hooks modules via package subpath exports (./hooks, ./hooks/*.js, ./hooks/*) so consumers can import XGleanOptions as documented.
• Make the export changes regeneration-safe via .speakeasy/gen.yaml (typescript.additionalPackageJSON).
• Harden src/hooks/x-glean.ts by removing the unsafe cast, adding runtime guards, and aligning X_GLEAN_INCLUDE_EXPERIMENTAL semantics to only enable the header when the env var is explicitly true (case-insensitive). Any other non-empty value disables/omits the header; empty/whitespace falls back to constructor options.
• Update README.md constructor options example to be TS-safe (satisfies SDKOptions & XGleanOptions).
• Expand unit tests for the new env semantics and runtime-guard behavior.

Why

After #96, TypeScript consumers could no longer pass excludeDeprecatedAfter / includeExperimental as constructor options without excess-property errors, and XGleanOptions was not importable from the published package. Also, env var parsing previously allowed accidental values like X_GLEAN_INCLUDE_EXPERIMENTAL=false to be sent as a header.

Test Plan

• npx vitest run src/__tests__/x-glean.test.ts
• npm run lint
• npm run build
• Verified import('@gleanwork/api-client/hooks/x-glean-options.js') resolves with the new export map.