* feat(core): write runtime.json sidecar for active sessions

Port kimi-cli PR #2082 part 1 to qwen-code.

On interactive session start, atomically write a small JSON sidecar at <projectDir>/chats/<sessionId>.runtime.json recording the (pid, session_id, work_dir, hostname, started_at, qwen_version) tuple.

External tools (terminal multiplexers, IDE integrations, status daemons) can map a running PID to its session id and work dir without parsing argv. Write is best-effort: a read-only filesystem must not block UI startup.

OS process title (was #3713) and dynamic OSC tab title (kimi #2083) remain out of scope.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix(core): refresh runtime.json on same-PID session swap

Config.startNewSession() reassigns this.sessionId in the same process,
which is reached by /clear, /reset, /new and /resume. Previously the
old <oldId>.runtime.json was left behind, falsely claiming the still-
live PID for a session no longer being served, and no new sidecar was
written for the incoming session.

Centralize the swap by clearing the old sidecar and writing a fresh
one for the new session id from inside startNewSession itself, so all
same-PID transitions are covered. The refresh runs as a fire-and-
forget best-effort; failures must not block the session switch.

Mirrors the post-merge Codex P1 fix on kimi-cli PR #2082 (the source
of the runtime.json sidecar pattern this PR ports).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix(core): only refresh runtime.json when this process owns it

Mirrors kimi-cli PR #2082 commit e237951f (Codex P1 r3158754463): a
short-lived non-interactive invocation (qwen --prompt, ACP, etc.) that
runs `/clear` would otherwise call `Config.startNewSession()`, delete
a concurrent shell's runtime.json sidecar (same outgoing session id),
and never write a replacement — leaving the shell discoverable to
nobody.

Add a `runtimeStatusEnabled` flag on Config, flipped on by the
interactive UI bootstrap immediately after the first successful
sidecar write, and gate the swap-time refresh in
`startNewSession()` on it. Non-interactive entry points never reach
the bootstrap, so they won't touch sibling sidecars.

Kimi later reverted the equivalent `write only from shell mode`
guard (commit 7083975a) in favor of writing from every long-lived
mode, but qwen's wire point is already interactive-only, so the
narrower guard is the right shape here.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…enLM#3713)