fix: close file handles via context manager in remaining call sites

[shaun0927]

Closes #96.

Problem

PR #76 wrapped the scheduler's open(...).read() in a context manager.
Several similar patterns elsewhere still rely on garbage collection to
release the underlying file descriptor. For long-running daemons
(tgapp.py, wechatapp.py, fsapp.py, etc.) the FD eventually leaks
to the per-process limit and the symptom shown in #48 ("multiple
calls then model unresponsive, needs restart") becomes hard to
diagnose because the actual EMFILE error surfaces in some unrelated
later open.

The recent refactor 47f106c also added a new unmanaged read at
agentmain.py:105 (/session.<key> = <file>), which would have grown
the leak surface.

Fix

Convert the remaining unmanaged open(...).<read|write>(...) patterns
to with open(...) as f: blocks. Same modes, encodings, bytes; only the
release timing changes.

File	Lines (pre-patch)	What
ga.py	23	code_run header copy
ga.py	387, 388	prepend mode in do_file_write
ga.py	424	_check_plan_completion read
agentmain.py	16	tools_schema load
agentmain.py	24	global_mem.txt seed
agentmain.py	27-28	global_mem_insight.txt seed (nested open().write(open().read()) — 2 leaks)
agentmain.py	33	tmwebdriver cdp_bridge config seed
agentmain.py	105	/session.<key>=<file> (added by 47f106c)
agentmain.py	242	reflect log append
frontends/wechatapp.py	183	decrypted media write to temp

Verification

$ python -m py_compile ga.py agentmain.py frontends/wechatapp.py
$ grep -nE 'open\\([^)]*\\)\\s*\\.(read|write)\\(' ga.py agentmain.py frontends/wechatapp.py | grep -v 'with '
(empty — all sites now use a context manager)

Scope choices

• Single janitor PR rather than splitting per file. The change is
  the same idiomatic conversion at every site, and reviewing them in
  one place makes it easy to confirm no behavior change.
• subprocess.Popen(stdout=open(...), stderr=open(...)) at
  agentmain.py:187-188 is intentionally not changed — those file
  objects are owned by the subprocess for its lifetime and closing them
  in the parent breaks the redirection.
• if sys.stdout is None: sys.stdout = open(os.devnull, ...)
  at lines 3 and 5 is also unchanged because sys.stdout keeps the
  reference alive for the entire process.
• open(_logf, 'a', ...) at the wechatapp __main__ block is
  unchanged for the same reason — the file is bound to sys.stdout
  for the lifetime of the daemon.

Net diff: +21 / -11 lines, no functional change.