Reconcile AGENTS/CONTRIBUTING docs with current config and auth behavior

[Copilot]

Nightly docs reconciliation flagged drift between AGENTS.md/CONTRIBUTING.md and the implemented behavior. The main gaps were token fallback priority clarity and missing quick-reference mentions for supported config fields and make help.

• Auth token priority alignment (AGENTS.md)

  • Clarified GITHUB_PERSONAL_ACCESS_TOKEN as the third-priority fallback.
  • Added missing GH_TOKEN as the lowest-priority fallback (GitHub CLI-provided token path).

• JSON stdin/TOML quick-reference completeness (AGENTS.md)

  • Added explicit mention of optional JSON stdin fields:
    • stdio: entrypoint, entrypointArgs, mounts
    • http: connectTimeout
  • Added direct pointer to full references for broader field coverage:
    • docs/CONFIGURATION.md
    • config.example.toml
    • includes gateway/top-level fields like keepalive_interval, sequential_launch, guards_mode.

• Make target discoverability (CONTRIBUTING.md)

  • Added make help to the documented command set so contributors can enumerate available targets directly.

- `GITHUB_PERSONAL_ACCESS_TOKEN` - Third-priority GitHub auth fallback
- `GH_TOKEN` - Lowest-priority GitHub auth fallback (set by GitHub CLI)
...
- **Note**: In JSON stdin format, stdio servers also support optional `entrypoint`, `entrypointArgs`, and `mounts` fields; HTTP servers support optional `connectTimeout`

[Copilot]

Pull request overview

Reconciles AGENTS.md and CONTRIBUTING.md with actual behavior surfaced by the nightly docs reconciliation: clarifies the GitHub token fallback priority, fills in missing JSON-stdin/TOML quick-reference fields, and adds make help to the contributor command list.

Changes:

• Clarify GITHUB_PERSONAL_ACCESS_TOKEN as third-priority and add GH_TOKEN as lowest-priority fallback, matching internal/envutil/github.go.
• Note additional optional JSON-stdin fields (entrypoint, entrypointArgs, mounts, connectTimeout) and point to docs/CONFIGURATION.md / config.example.toml for full coverage.
• Document make help in CONTRIBUTING.md.

Show a summary per file

File	Description
AGENTS.md	Aligns token priority docs with LookupGitHubToken and broadens JSON/TOML field references.
CONTRIBUTING.md	Adds make help to the documented build/dev command set.

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• Files reviewed: 2/2 changed files
• Comments generated: 0