6m0 clean up BASH_ENV context tempfile via traps on interrupt

source_template_file created its context tempfile with mktemp but only ran
rm -f on the success and explicit-failure paths. When a render was interrupted
by a signal (terminate_active_generation SIGTERMs the backgrounded render
subtree on Ctrl-C/abort) the tempfile leaked.

Register cleanup in exactly one place, inline in source_template_file's body:

- A RETURN trap covers normal and error returns and clears all of these traps
  (including itself) so it cannot linger and fire on an enclosing function's
  return against the out-of-scope context_file local (set -u). The trap must be
  set in the function body, not a helper: without functrace a RETURN trap is not
  function-scoped and would fire when a helper returns, deleting the file before
  the render runs.
- INT/TERM/HUP handlers remove the file, clear the traps and re-raise so the
  process still exits with the signal's default disposition. PIPE is not
  trapped (the internal context-build pipeline emits legitimate SIGPIPE);
  SIGKILL is untrappable, leaving only the OS-reaped KILL-escalation case.

The redundant rm -f calls are removed. Adds
test_template_interrupt_removes_context_file, which runs an in-flight render and
SIGTERMs it, asserting the context file is gone.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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