Add native grok agent backend

[falkoro]

What

Adds agent: grok — a native one-shot CLI backend for the standalone grok CLI (Grok Build), modeled on the existing claude/codex/pi pattern:

• internal/agent/grok.go: temp-file prompt (--prompt-file) so large review prompts never hit argv limits, plain-text output via --output-format plain, schema contract inlined in the prompt (same approach as pi), gitSafeEnv, classifyTransient retries, context cancellation during streaming.
• internal/types: AgentGrok; factory case in NewWithOptions; doctor listing.
• internal/config: agent_args_override allowlist entry + reserved flags — including --effort, which the grok API rejects (400) and must never be injected.
• cmd/fakeagent: grok handler for the e2e harness.
• Unit tests for construction, arg ordering (user extraArgs take precedence), schema prompt contract, and the absence of --effort.

Why

grok is a capable, cheap reviewer/writer; today the only way to route it through no-mistakes is via opencode with an xAI model, which adds a server dependency and breaks in two known ways (opencode serve rejects --model from agent_args_override, and xAI's 200-tool cap kills startup when global MCP servers are configured). A native backend removes that chain entirely.

Verification

• gofmt clean, make lint clean
• go test -race ./... — green except TestInitTracksCommandTelemetry, which also fails on a clean checkout of main in my environment (network-dependent)
• make e2e green (131s)
• Dogfooded: this branch is running as my daily gate (agent: grok + agent_path_override) reviewing a 108-file Astro monorepo diff.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds support for the Grok agent across configuration, detection, and execution, including a new agent implementation, fakeagent support, and unit tests.

Changes:

• Introduces AgentGrok and wires it into agent construction and defaults.
• Adds Grok CLI argument reservation/validation and includes Grok in doctor checks.
• Implements grokAgent plus fakeagent Grok support and tests for prompt/arg building.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
internal/types/types.go	Adds AgentGrok constant for agent enumeration.
internal/config/config.go	Registers Grok binary defaults, reserved args, and validation messaging.
internal/cli/doctor.go	Adds Grok to binary presence checks.
internal/agent/agent.go	Enables NewWithOptions creation for Grok and updates error message.
internal/agent/grok.go	Implements Grok agent execution, prompt/args building, and retries.
internal/agent/grok_test.go	Adds unit tests for Grok agent construction and helpers.
cmd/fakeagent/main.go	Routes fakeagent grok invocations.
cmd/fakeagent/grok.go	Implements fake Grok CLI behavior for e2e scenarios.

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[falkoro]

Rebased onto main (v1.29.0) and addressed the review feedback in 066fc6a:

• grok.go (stdout reading): replaced the bufio.Scanner + 256 MB max-token buffer with a bufio.Reader that reads incrementally via ReadString('\n'), so a single very long line no longer forces a large up-front allocation.
• grok.go (buildGrokArgs comment): corrected — managed flags are appended last and therefore take precedence (grok honors the last occurrence of a flag), and these flags are reserved in config so users can't supply them. The previous comment claimed the opposite.
• grok_test.go: dropped the developer-specific /home/falk/.grok/bin/grok paths in favour of a neutral /path/to/grok.

gofmt/go vet clean, go build and the agent tests pass.