fix(agent-context): stack-scope reference fragments (revert deploy-all regression)

assemble() was installing ALL language/framework reference fragments into every
consumer regardless of stack, contradicting the documented design (README + the
Stack.tokens "install-selection set" in types.ts). Effect: e.g. a Kotlin project
received csharp.md / python.md / typescript.md, and the SKILL.md "read every
references/*.md" footer then pointed agents at irrelevant-language instructions.

Restore stack-scoping: a language/framework fragment installs only when its token is
in the resolved stack; non-language fragments (capability-checklist, migration) are
universal and always install. The footer is now accurate (only stack-relevant
fragments remain). Update the two assemble tests that asserted deploy-all, and
regenerate the conformance goldens (out-of-stack fragments removed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FaRaYFjvWVV8D6h33ejj1m

Author: dmealing