feat: sticky session for multi-turn conversation continuity

[you922]

Summary

Adds an opt-in sticky session feature that binds a caller (identified by callerKey) to a specific upstream Windsurf account for the duration of a multi-turn conversation.

Problem

When using Claude Code, Cursor, or any multi-turn agent with WindsurfAPI, the account pool selects a different account for each request. The cascade_id from the previous turn is only valid on the originating account — switching accounts mid-conversation causes context loss (#93, #133).

Solution

New module src/account/sticky-session.js maintains an in-memory binding map:

• Key: (callerKey, modelKey) — model dimension prevents cross-model collision
• Binding created when a response completes successfully
• Binding checked on the next request before falling through to normal pool selection
• Auto-cleared when the bound account becomes unavailable (rate limited, banned)
• TTL-based expiry: 30 minutes by default
• LRU eviction when at capacity (default: 10,000)

Integration

• src/auth.js: getApiKey() now accepts optional callerKey parameter, checks sticky binding first
• src/handlers/chat.js: calls setStickyBinding() after successful response

Configuration (opt-in)

STICKY_SESSION_ENABLED=1        # default: 0 (off)
STICKY_SESSION_TTL_MS=1800000   # default: 30 minutes
STICKY_SESSION_MAX=10000        # default: 10000 max bindings

Backward Compatibility

• STICKY_SESSION_ENABLED defaults to 0 (off)
• callerKey is null by default in getApiKey(), existing callers unaffected
• No breaking changes to public API

Closes #93, #133