/new should re-display startup context (loaded resources, git state refresh)

[m-aebrer]

Summary

When a user runs /new to start a fresh session, the TUI only shows ✓ New session started. It does not re-display the loaded resources listing ([Context], [Memory], [Skills], [Prompts], [Extensions], [Themes]) that appears at initial startup, nor does it refresh the git repo state that's baked into the system prompt. This makes /new feel disconnected — users lose visibility into what context the new session has access to.

Current Behavior

• /new calls handleClearCommand() which resets the session, clears all UI containers, and renders only ✓ New session started
• The header (logo, keybinding hints, changelog) is cleared and not rebuilt
• showLoadedResources() is not called — no [Context]/[Memory]/[Skills]/etc. listing
• _gitRepoState (captured once in the AgentSession constructor) is not refreshed — the new session's system prompt contains stale branch/commit/PR info from when the process originally started

Proposed Behavior

• After /new, re-display the loaded resources listing (respecting quietStartup setting — show diagnostics even when quiet, full listing otherwise)
• Refresh _gitRepoState so the new session's system prompt reflects the current branch, recent commits, and open PRs (the user may have switched branches or made commits since the process started)
• Optionally re-display a minimal header (logo + version) to give visual grounding, but skip changelog/"What's New" (already seen)

Acceptance Criteria

• After /new, the chat area shows loaded resources ([Context], [Memory], [Skills], etc.) same as initial startup (controlled by quietStartup setting)
• After /new, _gitRepoState is refreshed from the current working directory so the system prompt for the new session is accurate
• The changelog/"What's New" section is NOT re-shown (it's a per-version notification, not per-session)
• The keybinding hints header is NOT re-shown (user already knows them)
• quietStartup setting is respected: if enabled, only diagnostics/conflicts are shown (same gating as initial startup)
• Extension session_switch event still fires as before (no regression)

Technical Notes

• handleClearCommand() at interactive-mode.ts:4958 — needs to call showLoadedResources() after clearing
• AgentSession.newSession() at agent-session.ts:1901 — needs to call getGitRepoState() to refresh _gitRepoState (note: switchSession/resume already refreshes it, so there's precedent)
• showLoadedResources() at interactive-mode.ts:1148 already respects quietStartup and has a force option — can be called directly
• Consider whether resources should be reloaded (resourceLoader.reload()) or just re-displayed. If the user added a new AGENTS.md or skill file since startup, reloading would be more useful. However, that's a larger scope change — re-displaying the current state is the minimum fix.