perf(agent-manager): Gemini discovery reads every chat file in full on each refresh #263

Problem

GeminiSessionLocator.discoverSessions (packages/agent-manager/src/providers/gemini/GeminiSessionLocator.ts) runs for Gemini processes not already cached in the registry.

• It walks every ~/.gemini/tmp/<id>/chats/ directory and readFileSyncs every chat file in full (readCandidateSession).
• It keeps all of their contents in contentCache at once.
• There is no date window, so every refresh costs O(total Gemini history) in both bytes read and peak memory.

Proposed approach

• Skip chats/ directories that can't belong to a live process, using the project-hash ↔ cwd mapping that is already computed (cwdHashMap), before reading any file.
• Consider only files whose mtime is at or after the earliest candidate process start time (with some slack).
• Extract projectHash and sessionId from a bounded head read where the format allows; otherwise cache the parsed metadata keyed by path, inode, size and mtime.
• Remove contentCache; matched sessions are parsed once and cached by the same key.

Acceptance criteria

• With 500 historical chat fixtures across 20 projects and 1 unmatched live process, discovery opens only files from that process's project whose mtime falls in the window.
• No full contents of unrelated chat files are kept in memory; contentCache is removed or bounded.
• After the first refresh, with unchanged files, a refresh reads 0 bytes of chat content.
• Existing Gemini detection tests pass with identical results.