Subtask identity: human key (e.g. BUILD-8.1) + deep link

JC 2026-08-23: every subtask needs its own deep-linkable identity. Mint gapless per-item sub-key at create (ITEM-KEY.N); existing subtasks get keys lazily (mint-on-read in repo layer, no hand migration); web renders each subtask row with its key as a copyable anchor deep link (/app/p/{k}/items/{key}#{subKey}) with scroll+highlight on load. Built TOGETHER with BUILD-24 (PATCH done/title + MCP tool + real checkboxes) — same entity, one contract change.