agent: raise default SPROUT_AGENT_MAX_OUTPUT_TOKENS to 32768

[tlongwell-block]

Problem

When the sprout-agent's per-response token cap fires inside a tool_use block (typical for large shell command values — e.g. file creation via heredoc), the provider returns the block with whatever input JSON parsed cleanly so far. That's often {} if the cap hit mid-value of the first field. The MCP then rejects the call with:

Tool call rejected: Mcp error: -32602: failed to deserialize parameters: missing field `command`

From the user's perspective the MCP "errors on file creation," but the trigger is response length, not quoting or content. The session loops because the model has no signal that its emission was truncated.

Reproduced empirically: command values up to ~5-7KB succeed; ~10KB+ trigger the truncation. Same MCP, same model, same harness.

Fix

Raise the default SPROUT_AGENT_MAX_OUTPUT_TOKENS from 4096 to 32768.

• Sonnet 4 and Opus 4 (the models offered in the ACP settings panel at crates/sprout-acp/src/acp.rs:1530-1531) both support up to 64K output tokens, so 32K leaves comfortable headroom.
• Providers bill on tokens generated, not on the cap, so normal-turn cost is unchanged — only the ceiling moves.
• The env override (SPROUT_AGENT_MAX_OUTPUT_TOKENS) still wins for any deployment that wants to pin a lower value.

Files

• crates/sprout-agent/src/config.rs — default literal.
• crates/sprout-agent/README.md — env-var table row + a one-line note explaining the value.

Verification

cargo build -p sprout-agent     # clean
cargo test -p sprout-agent       # 17/17 pass
cargo fmt -p sprout-agent --check
cargo clippy -p sprout-agent -- -D warnings

Lefthook pre-commit and pre-push hooks ran clean (rust-fmt, rust-clippy, rust-tests, plus the desktop/web/mobile checks).

Follow-ups (separate PRs, not blocking this one)

• Agent-side MaxTokens guard: detect stop_reason: max_tokens with non-empty tool_calls in crates/sprout-agent/src/agent.rs and refuse to forward the partial tool_use, returning a corrective synthetic tool-result instead. Defense in depth for sessions that still exceed the new 32K ceiling, or for deployments pinning the env override lower.
• write_file MCP tool in sprout-dev-mcp: takes {path, content} as plain JSON fields, sidestepping bash heredoc quoting footguns entirely (indented EOF, space-indent under <<-, unquoted-delimiter $var expansion). Removes the dominant "one giant command value" usage pattern that creates the truncation pressure in the first place.