feat(memory): add bounded cursor pagination for Memory entries

Feature description

Add an authorized, bounded Server query for Memory entries, implementing the directory portion of D2 in RFC #1455. Track this as a focused deliverable related to the broader capacity problem in #1321.

Problem and proposed solution

At aa697c5315204249e090acdf1a00d0582e851c5f, POST /v1/memory/entries/list accepts Scope, inactive-entry, and tag-filter inputs but has no page size or cursor. Its response contains the complete selected entries. The persistence implementation loads entry versions referenced by the complete Memory manifest.

Client-side pagination or slicing the fully assembled HTTP response leaves the Server work unbounded. Generic Artifact pagination also does not paginate entries inside one Memory Artifact.

Specify and implement a bounded query with an explicit default/maximum page size, deterministic ordering, an opaque continuation cursor, and clear consistency/error semantics. Agree on whether this is an additive operation or a versioned evolution; do not silently change existing clients from complete results to an undisclosed first page.

Acceptance criteria

• Publish request/response schemas, ordering, page-size bounds, end-of-list behavior, and cursor invalidation/expiry rules.
• Apply current authorization and supported inactive/tag filtering before page selection; continuation requests reauthorize access.
• Bind continuation to the relevant resource/query context. Reject tampered, mismatched, and invalid/expired cursors without revealing other Scopes or private data.
• Define concurrent append/revise/retire behavior: traverse a pinned revision or explicitly require restart when the consistency boundary changes. Do not silently skip or duplicate entries.
• Preserve exact Memory and entry/citation identities so a listed item can be inspected correctly.
• Bound entry-body loading and response assembly, not just the final response slice. State and measure any remaining whole-manifest cost; if it prevents the promised bound, agree the necessary storage/index work before claiming bounded browsing.
• Cover empty and multi-page results, the final page, inactive/tag filters, unauthorized requests, permission changes, invalid cursors, and concurrent mutations through observable behavior tests.
• Include a scale fixture or measurement demonstrating the declared bounds without freezing private call counts.
• Document compatibility/migration for existing clients; keep old full-list semantics explicit while supported. Regenerate OpenAPI-derived code and run contract/relevant behavior checks.

Alternatives considered

• Fetch everything and page in the UI: does not bound Server work or network payload.
• Truncate responses without continuation: loses data and misrepresents completeness.
• Reuse Artifact-directory pagination: addresses a different level of the data model.

Additional context

• Refs #1321, #1455 (D2/AC-19), #1428, and the Web viewer work in #1454.
• Current request/response contract.
• Current entry loading.
• Scope: entry directory queries. Memory history pagination, full storage-growth remediation, split/compaction policy, and Desktop UI are separate work.
• Open for community implementation in a focused PR targeting master. Coordinate shared cursor/consistency and compatibility choices with the history work.
• This gates complete Memory browsing, not Desktop's initial explicit-save/search/exact-detail workflow.

Desktop consumer: #1654.

Related history-pagination deliverable: #1657. Both issues cover focused portions of #1321 and should agree on shared consistency/compatibility decisions.

Start with the current request/response contract in openapi/powercontext.yaml around line 8248 and entry loading in src/powercontext/builtin/persistence/memory.py around line 181. Review related issues #1321, #1455, and #1657 before settling shared cursor, consistency, and compatibility decisions. Done means bounded, authorized pagination with observable behavior and scale tests, documented compatibility, regenerated OpenAPI code, and relevant checks passing.