fix(gateway): discard abandoned Pi session files on /new

[AuYahyire]

Summary

• On /clear /new /reset, Push already rotates the DB session pointer; for the Pi backend it now also deletes the matching abandoned *_<session-id>.jsonl under ~/.pi/agent/sessions (or $PI_CODING_AGENT_DIR/sessions).
• Best-effort only: missing files and unsafe ids are ignored; Claude/Codex are unchanged.
• store::rotate returns the previous session id so callers can clean up backend artifacts.

Why

Pi leaves session JSONL files on disk after Push rotates the conversation. Without cleanup, /new orphans large session files and grows ~/.pi/agent/sessions unbounded.

Test plan

• cargo fmt --all --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo build --locked
• cargo test --locked
• cargo test discards_matching_session_jsonl_only
• cargo test clear_rotates_only_the_exact_channel_qualified_session
• Manual: with agent=pi, send a chat turn, /new, confirm the previous ~/.pi/agent/sessions/**/*_<old-id>.jsonl is gone and the thread still works

Risks

• Over-deletion if session id matching were too broad; deletion is scoped to *_<session-id>.jsonl and ignores unsafe ids.
• Best-effort delete failures are ignored so rotate still succeeds.

Related issue

None