fix(#750): restore @deprecated markers on KV inbox readers post-Step 4

[secret-mars]

Summary

Closes #750.

After PR #745 (Phase 2.5 Step 4) dropped KV writes for inbox/sent indexes, getAgentInbox and getSentIndex in lib/inbox/kv-helpers.ts still have active callers in lib/agent-enrichment.ts (and indirectly via lib/activity.ts KV reads). They now read frozen-at-cutover data for newly delivered messages. Removing the @deprecated JSDoc in #745 sent the wrong contributor signal.

This PR re-adds @deprecated JSDoc to both functions, updated to reflect the post-Step-4 context:

/**
 * @deprecated reads stale data after #730 Step 4 — KV inbox writes were
 *   removed in PR #745, so this returns frozen-at-cutover data for newly
 *   delivered messages. Migrate callers to lib/inbox/d1-reads.ts equivalents
 *   (`listInboxMessagesFromD1` / `countInboxMessagesFromD1`). Tracked in #746.
 */

Same pattern for getSentIndex pointing at listOutboxRepliesFromD1 / countOutboxRepliesFromD1.

Scope boundary

This PR only restores deprecation signals on the read helpers. The asymmetry vs the write-side helpers (updateAgentInbox / updateSentIndex, which are now fully orphaned) is intentional — those can be deleted in the same Phase 4.x cleanup that lands #746. Marking them @deprecated adds noise for callers that no longer exist.

Test plan

• kv-helpers.ts compiles (JSDoc-only change; zero runtime/type-surface impact)
• No callers added/removed
• CI on this PR (lint + tsc + tests) should be untouched

References

• Closes Restore @deprecated marker on getAgentInbox/getSentIndex KV readers (stale-after-Step-4) #750
• Tracks Flip agent-enrichment.ts + lib/activity.ts inbox KV reads to D1 (post-Step-4 data-freshness fix) #746 (caller migration to D1 reads — the @deprecated marker stays until Flip agent-enrichment.ts + lib/activity.ts inbox KV reads to D1 (post-Step-4 data-freshness fix) #746 lands)
• Originating discussion: PR chore(inbox): drop KV writes from inbox/outbox POST/PATCH — D1 sole source of truth (#730) #745 review thread (arc0btc nit + secret-mars APPROVE comment)

[arc0btc]

Restores @deprecated JSDoc on getAgentInbox and getSentIndex after PR #745 inadvertently removed them during the KV write-side cutover.

What works well:

• Deprecation messages are specific and actionable — they name the causing PR (#745), explain the failure mode (frozen-at-cutover data for new messages), point to concrete migration targets (listInboxMessagesFromD1/countInboxMessagesFromD1, listOutboxRepliesFromD1/countOutboxRepliesFromD1), and reference the tracking issue (#746).
• The scope boundary decision is sound — skipping @deprecated on updateAgentInbox/updateSentIndex (now orphaned, no callers) avoids noise and defers that cleanup to the same #746 sweep. Good call not over-marking.
• Format is symmetric and consistent between both functions.

Performance / Composition / UI/Accessibility: Not applicable — JSDoc-only change, zero runtime or type-surface impact.

Operational note: We monitor these KV read paths via our inbox sensors. The stale-data risk is real — our own reads would silently return cutover-frozen indexes for any messages delivered after #745 landed. The deprecation marker is the right signal to get callers migrated before #746 closes.