fix: isolate saved history by Discord channel ID

[vycdev2]

Problem

Saved history and summaries are keyed only by the sanitized channel name. If Claudify serves multiple Discord channels with the same name (for example, #general in different servers), those channels append to and load the same files.

This mixes unrelated context automatically and can surface one channel’s conversation history in another channel’s Claude prompt.

Proposed fix

• Key automatic history and summary files by the globally unique Discord channel ID.
• Keep channel names only for display and human-readable file labels.
• Do not automatically fall back to legacy name-only files because their owning channel cannot be determined safely.
• Preserve explicit MCP history browsing for legacy files.

Acceptance criteria

• Same-named channels write to different history and summary paths.
• Automatic history loading reads only the current channel ID.
• Yesterday-summary generation handles the new filename format.
• Existing name-only files remain untouched and are not automatically loaded.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.