feat(tui): add Codex-style side conversations

[me2seeks]

Problem

Maka Desktop supports temporary Side Conversations, but the CLI/TUI does not expose the equivalent workflow.

The TUI can switch arbitrary Sessions through /session, including detaching from a running Turn, but it has no parent/side pairing, /side entry point, retained toggle, side-specific close behavior, or background parent status.

Users cannot ask a temporary contextual question while the main Turn continues and then switch directly between the two conversations.

Desired behavior

• /side opens a temporary side conversation from the latest settled Turn.
• /side <prompt> opens the side conversation and submits the prompt.
• The side Session inherits the parent model, thinking level, permission mode, collaboration mode, orchestration mode, cwd, and conversation prefix at creation.
• Parent and side Session configuration remain independent after creation.
• Parent and side Turns can continue independently.
• Only one TUI side conversation is retained at a time.
• Ctrl+/ switches between the side conversation and its parent without closing either.
• Ctrl+C in an empty side Composer closes the side conversation and returns to the parent. The key must never fall through to the global TUI exit path after a side-close attempt.
• Leaving the parent/side pair for an unrelated Session closes the temporary side conversation.
• Side Sessions remain hidden from ordinary Session discovery.
• Temporary-session cleanup remains recoverable after interruption instead of relying on renderer/TUI best effort.

Status line

Side-conversation status-line copy is always English.

Base state:

Side from main thread · Ctrl+/ to switch · Ctrl+C to close

Parent state variants:

• Side from main thread · main needs input · Ctrl+/ to switch · Ctrl+C to close
• Side from main thread · main needs approval · Ctrl+/ to switch · Ctrl+C to close
• Side from main thread · main failed · Ctrl+/ to switch · Ctrl+C to close
• Side from main thread · main interrupted · Ctrl+/ to switch · Ctrl+C to close
• Side from main thread · main closed · Ctrl+/ to switch · Ctrl+C to close
• Side from main thread · main finished · Ctrl+/ to switch · Ctrl+C to close

When the parent is visible and a retained side exists:

Ctrl+/ for side

A new parent Turn clears the previous terminal/actionable parent status.

Architecture constraints

• Runtime Host remains the sole authority for Session and Turn identity, conversation copying, execution, permissions, interactions, interruption, and removal.
• Reuse existing conversationCopy.intent, parentSessionId, branchOfTurnId, and mode:side_conversation; do not introduce another durable SidePair identity.
• TUI state is only an ephemeral navigation projection.
• Parent history is reference-only inside the side conversation and remains hidden from the side transcript.
• The workspace is shared live; the feature does not imply filesystem isolation.
• Parent model or permission changes do not propagate to an already-created side Session.
• Runtime Host policy and credential revocation still constrain every Session.
• Do not duplicate Desktop admission ownership work from fix(desktop): bind Side Conversation events to Host admission #3716.

Delivery slices

PR 1 — Host-backed TUI side lifecycle

Deliver one usable vertical slice:

• create the side copy from the latest settled parent Turn;
• expose /side [prompt];
• attach and render the side Session through the existing TUI Session path;
• close, interrupt if necessary, remove, and return to the parent;
• preserve recoverable cleanup semantics;
• keep side Sessions out of ordinary Session discovery;
• preserve parent draft and running Turn behavior.

This PR may consolidate the existing temporary-session cleanup seam if required, but must not add a speculative abstraction with only one real consumer.

PR 2 — Retained toggle and parent status

• retain the side while switching to its parent;
• add Ctrl+/ toggle, including terminal-compatible key normalization;
• observe background parent lifecycle and interaction state;
• add the English status-line contract above;
• close the side when navigating outside the pair;
• cover shortcut precedence, cleanup failure, background completion, approval/input priority, and session-switch races.

A third PR is allowed only if the first-principles review proves that cleanup ownership is an independently valuable migration whose separation produces a strict net reduction.

Required review gates before every commit

First-principles review

• What problem does this PR solve?
• How does this PR solve it?
• Is the problem defined correctly?
• Do the problem definition and solution follow first principles?
• Do they follow Occam's razor?

Adversarial and simplification review

• Are all changes in this PR the best solution to the stated problem?
• Does the result follow first principles and Occam's razor?
• Can low-quality tests be deleted?
• Can code or state be deleted?
• Can the implementation be refactored into a smaller and deeper module?
• Does simplify-audit identify any duplicate authority, representation, path, contract, or state?
• Has every retained abstraction passed the deletion test?

The review findings and resulting deletions or retained decisions must be recorded before committing.

Non-goals

• Multiple simultaneous TUI side conversations.
• Persisting or reopening a side conversation after a new TUI process starts.
• Synchronizing model or permission changes between parent and side.
• Promoting a side conversation to a durable ordinary fork.
• Adding a second status-line row.
• Localizing side-conversation status-line copy.

[me2seeks]

I'm working on this.