docs(go): recursive provider_state scoping and link config

[raphaelvigee]

Documents the two user-facing additions from hephbuild/heph#116.\n\n## What changed in heph\n\n1. recursive scoping knob for provider_state\n\nPreviously test = False silently cascaded to all descendant packages. It now applies only to the exact declaring package by default. Set recursive = True to restore subtree-wide behavior. The go_codegen_root and go_codegen_deps keys are unaffected — they always apply to descendants.\n\n2. link = {...} config for package main binary targets\n\nNew link struct on provider_state controls the go tool link invocation for build targets:\n- flags — passed verbatim before -o\n- deps — hashed inputs staged into the link sandbox\n- runtime_deps — staged at run time only, not hashed\n\nHonors the same recursive scoping; values accumulate shallow→deep across applicable states.\n\n## Docs changed\n\n- website/docs/plugins/go.md\n - Section header renamed to "per-package configuration" (was "per-subtree") to reflect the new default\n - Recognized keys table: added link and recursive rows; updated go_codegen_root/go_codegen_deps rows to note they are always subtree-wide\n - "Skipping tests" section: updated to show recursive = True is now required for subtree-wide disable\n - "Test environment" section: updated scoping note (struct form is also package-scoped by default, not uniquely so)\n - New "Link configuration" section with example and field table\n\n- plugins/heph-go/skills/heph-go/references/go-plugin.md\n - Same table and section updates as the docs page\n - Added link section\n - Added troubleshooting row for recursive being missing

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