[FEATURE]: navigate the transcript by prompt, landmark and block (ctrl+up/down, ctrl+shift+up/down, ctrl+alt+shift+up/down, ctrl+home/end) #53331

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

A long session can only be moved through by line, half page, page, or the whole transcript (home/end). messages_previous / messages_next exist, but they are unbound by default, and they skip any prompt within 10 rows of the viewport top. You also can't step through what the assistant did (its responses, reasoning or tool calls) or jump straight to the answer a turn ended with. Related requests were closed without an implementation: #34727 (jump between your own prompts) and #37699 (message-boundary scrollbar markers).

Proposal: three levels of navigation, bound by default and overridable in tui.json.

• messages_previous / messages_next (ctrl+up / ctrl+down): move to the previous or next user prompt, every one of them.
• messages_landmark_previous / messages_landmark_next (ctrl+shift+up / ctrl+shift+down): move to the previous or next landmark. Landmarks are user prompts, the final response of each turn (its last text part), and three kinds of tool call: question (what you were asked and answered), todowrite (the plan) and task (subagents). Edits, reads and shell calls are left out, because they are as dense as the blocks themselves.
• messages_block_previous / messages_block_next (ctrl+alt+shift+up / ctrl+alt+shift+down): move to the previous or next block, meaning each user prompt, assistant text part, reasoning part with content, and tool call. Reasoning without content is skipped. That covers encrypted blocks and the empty "Thinking" placeholders that GPT models emit on every step, which render as a bare Thought header and carry nothing worth stopping on.

A block lands at the top of the viewport, the same way messages_next places a prompt today. Past the last block the view returns to the bottom and follows new output again; before the first it goes to the top.

Also add ctrl+home / ctrl+end to messages_first / messages_last. Most terminal apps use those keys for "top" and "bottom", and plain home/end already do it here.