Move model effort chips to a fixed side column

[gaoyu06]

The composer model popover no longer inserts effort chips between rows (that reflowed list height and flickered). Efforts live in a reserved right column.

Scope

• Layout: agents | models | efforts (models | efforts when the agent is locked).
• Hover / keyboard still pick which model's chips show; command strings and lock behavior are unchanged.
• Extracted effortColumnIdx helper with unit tests.

Ship-ready

• pnpm test: 30 files / 471 tests (6 new).
• pnpm check: clean.
• No new dependencies.

Needs review

• Column width vs. long effort labels.
• Popover width on narrow windows.

Review preconditions

• Hovering models must not change row height or list jump.
• Chip click still selects model + effort in one step.
• Locked sessions still hide the agent rail.