mark legacy cofhejs pages with deprecation warning

[roeezolantz]

Summary

cofhejs is the legacy client library — superseded by @cofhe/sdk, with a full migration guide already at /client-sdk/introduction/migrating-from-cofhejs. The current cofhejs/* pages don't say anything about being legacy, so users landing there from search or external links have no idea the API has moved.

This is a minimal-risk fix: add a <Warning> callout to the top of every page under the cofhejs/ tree, pointing at the equivalent @cofhe/sdk page and the migration guide. The pages themselves stay (the tab remains in docs.json) so existing projects on cofhejs aren't stranded.

This covers gap A-6 from the docs audit on XDL-11.

Where the underlying change was made

• cofhejs was renamed and reshipped as @cofhe/sdk@0.1.0 — cofhesdk CHANGELOG 0.1.0 — `a83facb` & `8d41cf2`. Direct quote: "Prepare for initial release. Rename scope from @cofhesdk to @cofhe and rename cofhesdk package to @cofhe/sdk." Every release since has been on the @cofhe/* packages; the standalone cofhejs package on npm is frozen.

Changes

Same <Warning> block added at the top of all 9 cofhejs pages, with the per-page link aimed at the most relevant @cofhe/sdk replacement:

File	Cross-link target
cofhejs/introduction/overview.mdx	/client-sdk/introduction/overview
cofhejs/introduction/installation.mdx	/client-sdk/introduction/installation
cofhejs/introduction/mental-model.mdx	/client-sdk/introduction/mental-model
cofhejs/guides/encryption.mdx	/client-sdk/guides/encrypting-inputs
cofhejs/guides/error-handling.mdx	/client-sdk/guides/error-handling
cofhejs/guides/permits-management.mdx	/client-sdk/guides/permits
cofhejs/guides/sealing-unsealing.mdx	/client-sdk/guides/decrypt-to-view (sealing/unsealing is internal in the new SDK — decryptForView / decryptForTx handle it)
cofhejs/examples/end-to-end.mdx	/client-sdk/examples/end-to-end
cofhejs/resources/templates.mdx	/client-sdk/examples/templates

Every Warning also links to /client-sdk/introduction/migrating-from-cofhejs for the side-by-side mapping.

Out of scope (intentional)

• I did not remove the cofhejs tab from `docs.json`. The audit recommended this as a possible follow-up step, but it's a bigger UX call (does the team want to keep the tab visible at all?). The Warning is the lighter, reversible step.

Test plan

• `mint dev` renders each cofhejs page with the Warning rendered above the original first heading / paragraph.
• All 9 cross-links resolve to existing client-sdk pages.