Every ask row says what it waits for: one `waitingFor` word computed in one place, shown on /asks and returned by MCP `asks` #1825

Split from #1603. For: a person reads why a row is theirs without opening it.

Done looks like:

• [ ] One pure function (in src/web/studio/asks.ts or a new src/core/waiting_for.ts) gives each ask row one of: "ready to land", "conflicts with main", "a decision", "a review", "assigned to you", with whom it waits on.
• [ ] /asks shows it on each row (escaped); MCP asks returns waitingFor on each item.
• [ ] Nothing about which rows appear, their order, counts, dedupe or mutes changes in this issue.
• [ ] Tests in a new file named in GATE: each word for a fixture of each kind; MCP and /asks give the same word for the same row. Run test/asks.test.ts test/asks-expiry.test.ts test/asks-mute.test.ts test/asks-bulk.test.ts test/check-list.test.ts with it before pushing.