Cycle reasoning effort with Shift+Tab and show it on the prompt

[TheGreatAxios]

Summary

Rebinds Shift+Tab to cycle reasoning effort for the live model and keeps the current effort visible on the prompt border. Plain Tab still toggles prompt/transcript focus.

Closes CL-6298.

Changes

• cycleReasoningEffort walks the model's supported ladder and wraps
• Shell Shift+Tab / backtab handler invokes a host callback (does not steal focus)
• Runner rebuilds inference sources and refreshes the prompt model label on each cycle
• Status flash shows the new effort (or that the model has none)
• Docs/comments no longer claim Shift+Tab toggles auto mode

Test plan

• cycleReasoningEffort unit tests
• keybindings catalog probe for Shift+Tab
• pre-commit typecheck + build
• Manual: Shift+Tab cycles effort on a reasoning model; flash + border update
• Manual: Shift+Tab on a non-reasoning model flashes "no levels" and leaves config alone
• Manual: plain Tab still toggles focus

[linear-code]

CL-6298

[TheGreatAxios]

Reviewed as reviewer of record against CL-6298.

Verdict: approve (GitHub will not let this identity formally approve its own PR).

Shift+Tab cycles the live model's supported effort ladder, rebuilds inference sources so the next turn picks it up, and the prompt border label plus a status flash show the new value. Plain Tab still toggles focus. Overlays still treat unshifted Tab as complete.

Checked:

• cycleReasoningEffort wraps the per-model ladder; undefined/unsupported current starts at the first supported level; non-reasoning models return undefined and the runner flashes instead of mutating config.
• Shell onKey handles both tab+shift and backtab, after overlay handling, and does not steal unshifted Tab.
• Runner updates config.reasoningEffort, calls buildSessionSources(), agentProxy.setSources, setPromptModelLabel, and flashes the new level.
• Live keybinding probe in keybindings.test.ts asserts the handler fires and focus is unchanged.
• README / config comments no longer claim Shift+Tab toggles auto.

Nits (not blocking):

• docs/TUI.md is the normative TUI spec and still documents Tab without Shift+Tab. Help overlay is updated via keybindings.ts; TUI.md should match.
• docs/PRODUCT.md does not mention the new binding (README does).
• First press when session effort is unset jumps to supported[0] (e.g. minimal on gpt-5), not next-after-the-implicit-role-default.
• /model does not clamp a previously cycled effort onto the new model's ladder. Pre-existing, but Shift+Tab makes edge values (none, xhigh) more reachable.

Commit message is plain English and matches the change.