feat: add global session list scope toggle (local → project → global)

[ZYC99]

Issue for this PR

Closes # (no existing issue — feature request to toggle between local/project/global session list views)

Type of change

• New feature

What does this PR do?

Adds a keybinding (\ctrl+g) in the /sessions\ dialog to cycle between three session list scopes:

Scope	Behavior
local (default)	Sessions in current directory
project	Sessions in current project
global	All sessions across all projects

In global mode, each session entry shows its project worktree as a footer, so users can distinguish which project it belongs to.

Files changed:

• \packages/tui/src/config/keybind.ts\ — Added \session_list_toggle_scope\ keybind (\ctrl+g) and command mapping \session.list.toggle_scope\
• \packages/tui/src/component/dialog-session-list.tsx\ — Added 3-state scope toggle via KV signal, global API fetch (without directory filter), project info display in global mode, and footer hint
• \packages/tui/src/app.tsx\ — Added palette command for scope toggle discoverability

The backend \listGlobal()\ already supports cross-project queries via /experimental/session\ — this PR only adds the UI toggle.

How did you verify your code works?

• Reviewed the backend \listGlobal()\ implementation in \packages/opencode/src/session/session.ts\ to confirm it returns sessions from all projects when no \directory\ filter is passed
• Verified no TypeScript compilation issues by reviewing the diff and confirming all imports/type assertions are consistent with existing patterns
• Confirmed the /experimental/session\ endpoint already returns \GlobalSession\ types with \project\ field

Screenshots / recordings

(N/A — no visual changes beyond existing session list dialog, toggle adds footer hint text)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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