fix(agent_launch): headless prompt passed as a single argv string hits Linux's 128KB MAX_ARG_STRLEN (E2BIG) — pipe via stdin instead

[getappz]

Summary

Fixed the E2BIG bug in src/agent_launch.rs:

Root cause confirmed: headless_argv() appended the full built prompt as a trailing argv element, and run_captured()/run_headless() passed it to Command::args() — hitting Linux's MAX_ARG_STRLEN (128 KiB per-argv-string cap) once an item's comment history grew the prompt past that.

Fix:

• headless_argv() (agent_launch.rs:283) no longer appends the prompt — argv is now just [binary, ...print-mode flags, ...extra_args].
• run_captured() gained a stdin: Option<&str> parameter. When Some, it pipes stdin and writes the full prompt on a dedicated thread (mirroring the existing stdout/stderr reader-thread pattern) before dropping the handle to send EOF — avoiding the deadlock risk called out in the task.
• run_headless() now passes Some(prompt) through to run_captured.
• No caller-side changes needed — src/cli/work.rs:589 and src/agents.rs:233 both go through run_headless's unchanged public signature.

Verified stdin support per-agent (not from memory, per the task's ask):

• claude -p: live-tested against the installed binary — Error: Input must be provided either through stdin or as a prompt argument with no positional prompt/no stdin, and a real API round-trip succeeded when piping via stdin only.
• opencode run: live-tested — errors without a message when stdin is empty, but a piped stdin prompt with no positional message produced the expected model reply.
• gemini-cli: confirmed via source (packages/cli/src/gemini.tsx) — pushes stdin content as --prompt when no prompt arg is present.
• codex exec: confirmed via source (codex-rs/exec/src/lib.rs's resolve_root_prompt/read_prompt_from_stdin, StdinPromptBehavior::RequiredIfPiped), backed by the crate's own prompt_stdin.rs test suite.

All four agents support the stdin fallback, so no argv-based special-casing was needed.

Tests: updated the 6 existing run_captured call sites and the 2 headless_argv assertion tests for the new signatures, and added run_headless_pipes_a_prompt_over_the_argv_length_limit_via_stdin — a 200 KB prompt piped through run_headless into sh -p -c 'wc -c', asserting the full byte count arrives via stdin (this would have failed with E2BIG under the old argv-based path). All 24 tests in agent_launch::tests pass, plus the dependent cli::work:: and agents:: test modules. cargo clippy/cargo fmt clean on the changed file (one pre-existing, unrelated clippy failure in agent-registry/src/router.rs confirmed via git stash).

Committed as 6a5de72 on task/75.

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• src/agent_launch.rs

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