Build item_get and item_list v2 #313

Read tools from #309. Includes the paging contract of #304 and the partial reads of #305. Built on the existing data model behind one history interface, so #320 swaps only the storage. Start from the descriptions measured in tests/Conderis.Tests/ToolSurface/V2Prototype.cs.

item_get

• number or ref (instructions, memory:<id> for personal memories, proposal:<id>); projectId optional for personal memories.
• Returns the item, its direct relations as short entries and its version. No depth.
• history: true returns history metadata per version (version, author, time, changed fields); version reads the text as it was at that version. No revision id in the response.
• offset/take per #305.

item_list

• kind (task, memory, page, proposal), query, filter object per kind, sort, limit, cursor; response with total, hasMore, nextCursor.
• Tasks are the active ones (open, in_progress, blocked) unless includeClosed; no status filter (#311).
• Filters per kind: task priority, assignee, parent, includeClosed; memory type, tags, visibility, includeArchived, includeExpired; page parent, includeArchived; proposal state, author, targetKind.

Done (113f523)

src/Conderis/Services/Items: ItemReadTools (not registered as MCP tools until #317), IItemHistory with RevisionItemHistory, TextPart, ItemPage; McpJson.Node removes nulls from hand-built JSON but keeps a proposal's changes and current untouched. Tests in ItemReadToolsTests; BuiltV2ToolsMatchTheMeasuredSchemas fails if a shipped description drifts from the one measured in #311.

Decisions taken while building:

• Version until #320: for memories, pages and instructions version is the position of the published revision (1, 2, 3 …), found by following each revision's base revision because creation times can be equal. Moves and archiving change no version, as decision 7 of #309 wants. Tasks keep their version column. #315 maps a written version to the matching revision the same way.
• Earlier task versions are not rebuilt: version on a task is refused with a pointer to history=true, whose entries carry each field's old and new value. #320 stores full states and lifts this.
• Paging runs over the complete, deterministically ordered list in the tool; KnowledgeService.List got a limit parameter and ProposalService a CountPublic for this. Cursors are bound to caller, kind, project, query, filter and sort; a cursor from another list is refused.
• Partial reads cut at the last line break before take and never inside a surrogate pair; a partial answer carries contentLength, offset, nextOffset, complete: false and the Markdown outline (headings inside code fences excluded).
• Pages are listed in tree order; sort is refused for them, and query for proposals.

Open for later subtasks: page history names human authors by email, as item_history does today; hiding that belongs with #316's rule of no email addresses in responses. The old read tools and their tests stay until #317 removes them.