docs(5/10): consolidate extension and target contracts

[flyingrobots]

Decision

Make the topic tree self-sufficient for project manifests, directives, emitters, target descriptors, and the post-Node extension boundary.

This is documentation reconstruction slice 5 of 10.

Scope

Rewrite current, tested contract truth into:

• docs/topics/project-manifests.md
• docs/topics/directives.md
• docs/topics/emitters.md
• docs/topics/extension-modules.md
• docs/topics/compiler-boundary.md
• docs/topics/artifacts-and-cache.md
• docs/topics/legacy-node.md

Extract from and then delete the superseded reader-facing material under:

• docs/reference/ for project manifests, directives, external targets, and extension generation;
• docs/guides/ for extending, generators, and module authoring;
• docs/hosts/;
• docs/build-artifacts.md;
• docs/LEGACY_NODE_MIGRATION.md;
• directly overlapping architecture/module-contract pages.

Preserve the distinction between the shipped target verify descriptor validator, embedded Rust generation examples, and future external target execution. Do not resurrect arbitrary module command loading. Historical Node command rewrites belong only in the concise legacy topic.

RED

Add focused examples that fail when docs claim nonexistent target execution, an unsupported descriptor field, a dead package path, or a retired Node command as current behavior.

Acceptance

• Each extension task has one current topic and runnable/inspectable evidence.
• Descriptor verification and execution status are unambiguous.
• Module ownership remains domain-empty and external.
• Source guides/references are deleted after migration; no archive directory is created.
• Active machine policy such as the Node-retirement JSON ledger is not deleted in this slice; its relocation is handled by slice 9.
• Docs truth, extension fixtures, preflight, and git diff --check pass.

Scheduling

• Ordered after docs(4/10): rewrite compiler architecture from shipped Rust #746.
• Blocked by refactor(governance): replace goalposts with version milestones #742.
• The v0.3.0 label is temporary until the plain version-milestone cutover.

[coderabbitai]

🔗 Related PRs

#389 - feat(echo): implement complete E0–E4 roadmap (22/22 tasks) [merged]
#475 - Merge-worthy unmerged branches into omnibus reconciliation branch [merged]
#650 - feat(platform): add v0.2.0 project manifest platform [merged]
#686 - docs: add plain Wesley onboarding path [merged]
#692 - docs: add first contributor fast paths [merged]

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