fix: archive old transcript files on /new and /reset

[mcaxtr]

Fixes #14869

Summary

• Extract shared archiveSessionTranscripts() helper in session-utils.fs.ts that iterates transcript candidates and archives each with archiveFileOnDisk()
• Archive old transcript in sessions.reset RPC handler (TUI/webchat path) after creating the new session entry
• Archive old transcript in initSessionState() (chat channel path) after the session store update when /new or /reset triggers a reset
• Refactor sessions.delete RPC handler to use the same shared helper (deduplication)

Test plan

• archiveSessionTranscripts() archives existing transcript file and returns archived path
• archiveSessionTranscripts() archives transcript found via explicit sessionFile path
• archiveSessionTranscripts() returns empty array when no transcript files exist
• archiveSessionTranscripts() skips nonexistent files and archives only existing ones
• All 4 tests fail before the fix, pass after
• pnpm build && pnpm check passes

Greptile Overview

Greptile Summary

This PR adds a shared archiveSessionTranscripts() helper to archive on-disk session transcript .jsonl files and wires it into the session reset flows (RPC sessions.reset and chat-channel /new//reset handling) so old transcripts don’t accumulate. It also refactors sessions.delete to use the same helper, keeping transcript archiving behavior consistent across reset and delete paths.

The helper builds transcript path candidates from the session store location, explicit sessionFile, agent transcript location, and the fallback ~/.openclaw/sessions path, then best-effort renames any existing files to .<reason>.<timestamp> suffixes. Tests were added to validate the helper’s behavior across existing/missing files and explicit sessionFile paths.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk.
• Changes are localized, use synchronous best-effort file renames for transcript archiving, and are covered by new unit tests; I did not find any introduced functional regressions in the reviewed paths.
• No files require special attention

_{Last reviewed commit: 00b6333}

Context used:

• Context from dashboard - CLAUDE.md (source)
• Context from dashboard - AGENTS.md (source)

[gumadeiras]

Merged via squash.

• Prepared head SHA: 4724df7
• Merge commit: 31537c6

Valeu @mcaxtr!