fix(tools): don't compound-rewrite spawn_via_env background wrappers

[kshitijk4poor]

Summary

Salvage of #33756 by @charzhou — fixes background tasks failing to launch on non-local terminal backends (SSH/Docker/Modal/Daytona/Singularity). Extracted as a focused, one-concern PR from the original multi-feature branch (the unrelated image_gen / docker-media changes are intentionally not included).

The bug

Background tasks on non-local backends go through ProcessRegistry.spawn_via_env, which builds a hand-crafted, shell-safe wrapper:

mkdir -p T && ( nohup bash -lc CMD > LOG 2>&1; rc=$?; printf '%s\n' "$rc" > EXIT ) & echo $! > PID && cat PID

BaseEnvironment.execute() unconditionally ran _rewrite_compound_background on every command — including this wrapper. That rewrite (which defuses the A && B & subshell-wait trap for user commands) turns ( ... ) & echo $! into:

... && { ( ... ) & } echo $!
                   ^^^ `} echo` — no separator → bash syntax error

The wrapper then never produces a PID, the redirected output file is never created, and the agent sees an immediate exit code -1. This breaks every background launch on a non-local backend (e.g. a simple count-and-redirect script over SSH), which is why it reproduces on trivial setups.

Fix

• Add rewrite_compound_background: bool = True to BaseEnvironment.execute() (and the BaseModalExecutionEnvironment override, which accepts and ignores it). Default preserves existing behavior; the user foreground terminal path still rewrites.
• spawn_via_env passes rewrite_compound_background=False so its already shell-safe wrapper is left intact.
• Treat a wrapper that produces no PID as a failed launch (mark the session exited with a real exit code instead of exposing a fake running session), and don't register/checkpoint a session that never started.

Changes vs #33756

• Scoped to the background fix only (tools/process_registry.py, tools/environments/base.py, tools/environments/modal_utils.py + tests). The image_gen base_url/key_env overrides and docker-media-path changes from the original branch are dropped — those are separate concerns.
• Removed a duplicate session.exited = True in the PID-failure guard.
• Added a regression assertion that a failed launch is not added to _running.
• Added @CharZhou to scripts/release.py::AUTHOR_MAP so release contributor attribution resolves the co-author trailer.

Testing

tests/tools/test_process_registry.py -k spawn_via_env   → 3 passed
tests/tools/test_terminal_compound_background.py        → 34 passed
ruff check (changed files)                              → clean
ty (changed regions)                                    → no new diagnostics

Verified empirically with the exact wrapper shape: skipping the rewrite produces valid bash that launches the process, captures the PID, and writes the log/pid/exit files; the old rewritten form fails bash -n with a syntax error.

Based on #33756 by @charzhou.

[alt-glitch]

Supersedes the background-fix portion of #33756 (scoped to spawn_via_env compound-rewrite only, excludes unrelated image_gen/docker-media changes). Also supersedes closed #33754.