[cherry-pick] sessions: surface archived state in session header

[vs-code-engineering]

Cherry-pick of #320392 from main.

What

In the Agents window, an archived ("done") session is currently almost invisible — the only signals are missing actions and the Restore button hidden away in the title bar's command center.

This PR consolidates the archived-session UI on the session header:

• Distinct "done" glyph in the status icon. getStatusIcon now returns Codicon.passFilled (in the muted read-indicator color) for archived sessions, instead of reusing the plain read dot. Applies wherever the shared status icon is used (header, sessions list, picker).
• Restore action moved to the session header toolbar. Previously contributed to Menus.CommandCenter (title bar pill area); now contributed to Menus.SessionBarToolbar, navigation group (order 5), so it sits where New Chat would otherwise be. Gated by the session-scoped SessionIsArchivedContext, which is the right scope for a per-session header toolbar.
• Hide New Chat and Pin/Unpin in the header when archived. These actions don't apply to a done session — SessionIsArchivedContext.negate() added to both when clauses.

Why

A done session now reads as done at a glance (check glyph in the header), and the Restore affordance lives next to the session it acts on, rather than across the window in the title bar. The header toolbar stays focused on actions that are actually meaningful for the current state.

Notes for reviewers

• No new menu IDs or context keys — reuses the existing per-session SessionIsArchivedContext already bound in sessionView.ts (and used elsewhere for header context-menu gating).
• The Restore action still appears in the sessions-list per-item toolbar and the list item context menu (unchanged); only the title-bar command-center contribution moved.
• Title text is intentionally not dimmed — the glyph + hidden-actions combination conveys the state without changing typography.
• Type-check (compile-check-ts-native) and layers check both pass.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@sandy081

Matched files:

• src/vs/sessions/services/sessions/browser/sessionsListModelService.ts

@lszomoru

Matched files:

• src/vs/sessions/services/sessions/browser/sessionsListModelService.ts