fix discovery collector result metadata

[PrashantBtkl]

What changed

Discovery inventory results now expose trustworthy per-collector execution metadata while preserving the existing collector output wire format.

• Capture and return SSH command exit codes, including meaningful non-zero codes such as 100.
• Detect and report stdout/stderr truncation at the configured output cap.
• Keep command timeouts and other execution failures in collector_errors without reporting them as ordinary exit codes.
• Document the new additive response fields.
• Add tests for exit codes, timeout separation, exact output-cap boundaries, JSON serialization, and stream draining.

Why

Collector exit status was discarded, forcing packs to append fragile echo "rc=$?" workarounds. Output truncation was also silent, making incomplete package inventories look authoritative.

Fixes #137
Fixes #138

Validation

• GOCACHE=/tmp/forager-go-cache go test ./...

[gemini-code-assist]

Code Review

This pull request enhances the discovery executor to report collector exit codes, output truncation status, and output limit bytes, updating the documentation and adding corresponding tests. The feedback suggests sanitizing cfg.maxOutputBytes to prevent a potential slice bounds out of range panic if configured with a negative value, and offers a refactoring suggestion to simplify the exit code extraction logic for better readability.