fix(prompt): use mode-aware setup guidance for Web UI vs TUI

[LikiosSedo]

Problem

The system prompt in buildSreSystemPrompt() unconditionally recommended the /setup command for all configuration tasks — but /setup is TUI-only. Web UI users were incorrectly guided to use it instead of the sidebar Settings pages, causing confusion (agent would say "use /setup" while also saying "I'm in a Web UI session").

Solution

Make all setup/configuration guidance in the system prompt mode-aware (mode === "cli" vs web/channel):

• Environment & Configuration section: TUI → /setup command, Web → sidebar Settings → Credentials / Models
• No-credentials guidance (credential_list returns empty): TUI → /setup → Credentials → Add, Web → Settings → Credentials page
• Paste-credential redirect: TUI → /setup → Credentials, Web → Settings → Credentials page

Test plan

• Start a Web UI session, ask "how to configure cluster" — should reference sidebar Settings → Credentials, no mention of /setup
• Start a TUI session, ask the same — should reference /setup command as before
• In Web UI with no credentials, verify credential_list empty path guides to Settings → Credentials

[nightmeng]

LGTM. Clean fix for the mode-aware prompt guidance.

One minor note:

channel mode falls into the web branch — the ternary mode === "cli" ? ... : ... means channel sessions also get "sidebar Settings → Credentials" guidance. Verify this is correct for channel mode (e.g., Slack/DingTalk integrations). If channels don't have a sidebar, they might need their own guidance text. If channel sessions always go through the Web UI for setup, then the current behavior is fine.