fix(web): keep mobile session header compact

[techotaku39]

Summary

• Hide the reasoning ... label in SessionHeader below the sm breakpoint.
• Keep the label visible in the desktop header.
• Keep the composer StatusBar reasoning label unchanged on every viewport.

Problem

The reasoning label added in #1016 can wrap onto a third header line on narrow mobile screens. This makes the session header noticeably taller, while the same reasoning value is already visible in the composer status bar.

Using the existing Tailwind sm breakpoint keeps the mobile header to its intended title + metadata layout without removing the desktop status information introduced by #1016.

Test plan

• bun typecheck
• bun run test:web (158 files, 1279 tests)
• Verify on a narrow mobile viewport that SessionHeader no longer shows reasoning ... or gains a third line.
• Verify the composer StatusBar still shows the active reasoning effort on mobile.
• Verify SessionHeader still shows the reasoning effort at desktop widths (sm and above).
• Smoke-test the production Web/Hub build on ports 3016 and 3006.

Related

• Follow-up to fix(web): show Codex reasoning effort in SessionHeader #1016
• Related issue: web(codex): SessionHeader missing reasoning effort; create page missing Fast/Personality/Plan #1015

AI disclosure

This change was prepared with OpenAI Codex (gpt-5.6-sol).

[github-actions]

Findings
No findings.

Summary
Review mode: initial
No high-confidence issues found in the one-line web diff. Residual risk: I did not run tests or viewport checks in automation; PR body reports bun typecheck, bun run test:web, and manual mobile/desktop checks.

Testing
Not run (automation).

HAPI Bot