[FEATURE] VS Code Extension: Add setting to configure "Past Conversations" time limit

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The VS Code Extension's "Past Conversations" panel has a hardcoded time limit (~30 days from creation date) that cannot be configured. Conversations older than this limit disappear from the UI even though:

1. The JSONL files still exist and are intact in ~/.claude/projects/
2. The conversations are fully accessible via claude --resume <session-id> in terminal
3. Third-party extensions (like Claude Code Assist) can read and display these conversations

I'm aware that cleanupPeriodDays exists in ~/.claude/settings.json and controls file retention for the CLI. However, this setting has no effect on the VS Code Extension's "Past Conversations" filter. The extension should respect this setting or have its own equivalent.

Proposed Solution

Option A: Make the VS Code Extension respect cleanupPeriodDays from ~/.claude/settings.json

Option B: Add a dedicated extension setting:
"claudeCode.pastConversationsLimit": 90 // days, or 0 for unlimited

Option C: Allow resuming any conversation by session ID directly in the extension UI (similar to claude --resume <session-id>)

Alternative Solutions

Current workaround requires:

1. Using third-party extensions (Claude Code Assist) to find the session ID
2. Running claude --resume <session-id> in terminal
3. Working in terminal instead of the VS Code Extension UI

This defeats the purpose of having a VS Code Extension.

Priority

Medium - Would be very helpful

Feature Category

Configuration and settings

Use Case Example

I'm working on a long-running documentation project. The conversation started on November 1st, 2025, and contains:

• 1072 messages
• 82 file changes
• Accumulated context about project structure, decisions made, and pending tasks

Step-by-step scenario

1. November 3st: I start a conversation in VS Code Extension to work on documentation
2. November month: I continue working, the conversation appears in "Past Conversations"
3. December 4th (today): The conversation disappears from "Past Conversations" (older than ~30 days from original creation in early November context)
4. I check ~/.claude/projects/ - the JSONL file (9.4MB) is there.
5. I run claude --resume <session-id> in terminal - it works.
6. But I cannot access it from the VS Code Extension UI anymore

Expected: If terminal can show and resume the conversation, the VS Code Extension should too.

Additional Context

• The third-party extension "Claude Code Assist" (agsoft.claude-history-viewer) can read and display all conversations from ~/.claude/projects/ without any time limit, proving this is a UI filtering decision that helps me to find my conversation.
• Related issues: #9898 (conversation indexing inconsistencies between interfaces), #4172 (conversation retention settings)
• The cleanupPeriodDays setting in ~/.claude/settings.json controls file deletion but doesn't affect the VS Code Extension's UI filter.