connect: nest tool calls under the message that made them

[hbrooks]

Summary

Pressing up in the chat used to land on a tool call, which reads as a turn in the conversation when it is really work the agent did mid-message. Now a tool call belongs to its message's block.

Three levels, each opened by → on the level above:

● the message            a stop; ↑/↓ walk these
  ⎿ Ran 3 tool calls     part of the message's block
    ● Bash(pytest)       a stop once the message is opened
      ⎿ output           part of that call's block

So ↑ lands on the message with its tool chatter in tow, → reveals the calls and ↑/↓ step through them one at a time, → on a call opens its output, and ← walks back out a level.

Each lifted block also gets a blank tinted row above and below, shared between a message and the tool run attached under it rather than one pad each.

Rebased onto main, so the variable changes that shipped in #85 and #86 are no longer part of this branch.

Test plan

• tsc --noEmit clean, 381 tests pass (on @ellipsis-dev/sdk 0.5.0, which main picked up)
• New layOutItems coverage: a call belongs to its message block, revealed calls become their own stops, results travel with their call, ctrl+r reveals everything
• Drove the real arrow keys through an offline Ink render harness: ↑ highlights the message with ⎿ Ran 2 tool calls inside it; → expands to ● Bash(npm test) / ● Read(src/index.ts); ↑/↓ step between the two; → un-clamps the Bash output; ← re-clamps, ← again returns to the message