[FEATURE] Add context window usage indicator and warning before auto-compaction

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

When a session's context window approaches its limit, Claude Code silently triggers
auto-compaction with no prior warning to the user. From the user's perspective,
the conversation history simply disappears — with no countdown, no icon, no alert.
This is especially disruptive for long, multi-step project sessions where losing
context mid-work causes real productivity loss and confusion.

Proposed Solution

Expected Behavior

• A visible indicator (progress bar, icon, or badge) showing current context usage
• A warning notification when context is near saturation (e.g., at 70–80%)
• Optional: prompt the user to save a handoff note or summary before compaction occurs

Current Behavior

No indicator or warning exists. Auto-compaction happens silently.

Why This Matters

Claude Web retains conversation history across sessions. Users migrating from
Claude Web to the desktop app expect similar reliability. Silent context loss
without warning breaks trust and discourages long-term project use.

Alternative Solutions

_No response_

Priority

Critical - Blocking my work

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_