docs: preserve complete agent runtime architecture package

[jmagar]

Summary

• preserve the complete proposed Soma agent-runtime documentation package
• add specs, contracts, models, Rust-facing type blueprints, JSON Schemas, examples, implementation phases, code skeletons, file plan, and test matrix
• add the generated package manifest, validation report, and canonical validation script
• index the package from docs/CLAUDE.md and record it in CHANGELOG.md
• use neutral devhost fixture identities rather than internal hostnames
• narrowly exclude the schema-backed snippet and isolated external-consumer fixture from repository rules that do not apply to their artifact formats
• exact-pin soma-ops schemars to the already locked 1.2.1 release and refresh the external-consumer fixture lockfile

Validation

• check-agent-runtime-docs.py: 75 files and 7 schema-backed examples passed
• exact pinned fleet repository contract: passed
• root and external-consumer cargo metadata --locked --no-deps: passed
• git diff --check: passed
• cargo xtask check-stale-claims: passed
• cargo xtask check-schema-docs --check: passed
• cargo xtask check-architecture: passed (40 workspace packages, 92 internal edges)
• cargo xtask check-docs: generated docs current and Python platform policy passed; unrelated SDK import timing gate exceeded its 500 ms budget during the captured run

Scope

This PR preserves a proposal package for review. It does not claim the described runtime is implemented and should not be auto-merged solely as cleanup.

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