fix: archive old transcript files on /new and /reset (#14949)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: 4724df7dea247970b909ef8d293ba4a612b7b1b4
Co-authored-by: mcaxtr <7562095+mcaxtr@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

CHANGELOG.md

@@ -52,6 +52,7 @@ Docs: https://docs.openclaw.ai
 - Outbound/Threading: pass `replyTo` and `threadId` from `message send` tool actions through the core outbound send path to channel adapters, preserving thread/reply routing. (#14948) Thanks @mcaxtr.
 - Sessions/Agents: pass `agentId` when resolving existing transcript paths in reply runs so non-default agents and heartbeat/chat handlers no longer fail with `Session file path must be within sessions directory`. (#15141) Thanks @Goldenmonstew.
 - Sessions/Agents: pass `agentId` through status and usage transcript-resolution paths (auto-reply, gateway usage APIs, and session cost/log loaders) so non-default agents can resolve absolute session files without path-validation failures. (#15103) Thanks @jalehman.
+- Sessions: archive previous transcript files on `/new` and `/reset` session resets (including gateway `sessions.reset`) so stale transcripts do not accumulate on disk. (#14869) Thanks @mcaxtr.
 - Signal/Install: auto-install `signal-cli` via Homebrew on non-x64 Linux architectures, avoiding x86_64 native binary `Exec format error` failures on arm64/arm hosts. (#15443) Thanks @jogvan-k.
 - Discord: avoid misrouting numeric guild allowlist entries to `/channels/<guildId>` by prefixing guild-only inputs with `guild:` during resolution. (#12326) Thanks @headswim.
 - Config: preserve `${VAR}` env references when writing config files so `openclaw config set/apply/patch` does not persist secrets to disk. Thanks @thewilloftheshadow.