Add master engineer onboarding guide, prune stale docs

[macanderson]

Summary

Adds a single, plain-English docs/GUIDE.md for engineers onboarding onto this codebase, and removes stale exploratory docs from docs/.

What changed

• New docs/GUIDE.md: one master doc, in this order:
  1. Principles — the coding/design rules this codebase actually follows (provenance, budget honesty, consent, conformance-as-a-test, citation, version stability, unknown-field tolerance, content-as-evidence, and contribution conventions), each tied back to the ADR or mechanism that enforces it.
  2. Schema — the wire messages (handshake, query, frames, verify, error, ...), the frame shape, and every exported Rust type in contextgraph-types, described in plain terms.
  3. ADR index — a summary table for all 8 ADRs with links to the full text.
     Written to be readable with no prior context (short sentences, concrete examples, no unexplained jargon).
• Removed stale/exploratory content that was never part of the shipped protocol: docs/sketches/* (7 files, all deferred/never-implemented feature sketches), docs/future/context-receipt-impact-trace/* (out-of-scope downstream product), and docs/adaptive-context-reconciliation.md (a resolved, point-in-time reconciliation table).
• Repointed the now-dangling references to the removed files in SPEC.md, docs/adr/0004-dead-capability-surface.md, docs/adr/0007-protocol-product-boundary.md, docs/profiles/context-exchange-provider.md, and contextgraph-trace/README.md — kept as prose notes ("a design sketch existed here, since removed") rather than broken links. Left CHANGELOG.md/MIGRATION.md untouched since those are append-only historical logs.
• Linked the new guide from docs/index.md and README.md.

Checklist

• One logical change per PR (docs-only)
• Gate is green locally — fmt, clippy -D warnings, test (docs-only change, no code touched)
• A witness test is included, or a reason there isn't one is stated below — not applicable, docs-only
• Docs updated in the same PR if behavior or flags changed (README.md, docs/, doc comments, --help text)
• All commits signed off (git commit -s, DCO)
• CHANGELOG.md updated under [Unreleased] if user-visible — not applicable, internal docs reorganization, no user-visible behavior change

Registry submission (only if adding a row to docs/registry.md)

• Not applicable — this PR does not add/change a conformance registry entry

Protocol-stability impact (if a spec/wire change)

• Not applicable — no wire or spec change (SPEC.md edits are dead-link cleanup only, no normative text changed)

License

By submitting this pull request, I agree to dual-license this contribution under MIT OR Apache-2.0, as certified by my DCO sign-off.

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Generated by Claude Code

Summary by Sourcery

Consolidate engineer onboarding documentation and remove obsolete design material without changing the protocol or code.

New Features:

• Add a plain-English engineer onboarding guide covering project principles, protocol schema, Rust types, and accepted ADRs.

Bug Fixes:

• Replace references to deleted design documents with explanatory prose so documentation no longer contains broken links.

Enhancements:

• Prune stale exploratory, deferred, and resolved documentation that is not part of the shipped protocol.
• Link the engineer guide from the repository README and documentation index.

Documentation:

• Consolidate onboarding and protocol orientation material into docs/GUIDE.md while preserving links to the normative specification and follow-up documentation.

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[sourcery-ai]

Reviewer's Guide

This docs-only PR introduces docs/GUIDE.md as the canonical plain-English onboarding path, consolidates protocol and architecture guidance, removes stale exploratory material, repairs its references with historical notes, and links the guide from the main entry points.

File-Level Changes

Change	Details	Files
Created a single onboarding guide that consolidates implementation principles, protocol schema, Rust type mapping, and ADR navigation.	Documents enforcement mechanisms for provenance, token accounting, consent, conformance, versioning, tolerance, content safety, error handling, and protocol boundaries. Summarizes core envelope messages, frame representations, exported contextgraph-types types, and the optional lifecycle-record profile. Adds concise summaries and links for all eight accepted ADRs, plus contributor workflow guidance.	docs/GUIDE.md
Removed obsolete design and exploratory documentation that no longer represents shipped protocol behavior.	Deletes seven deferred feature sketches, the out-of-scope context-receipt impact-trace design, and the resolved reconciliation table. Preserves historical changelog and migration records.	docs/sketches/attribution-feedback.md docs/sketches/context-neighbors.md docs/sketches/host-trace.md docs/sketches/push-invalidation.md docs/sketches/query-filters.md docs/sketches/resolve.md docs/sketches/write-path.md docs/future/context-receipt-impact-trace/context-receipt-impact-trace-design.md docs/adaptive-context-reconciliation.md
Replaced broken links to deleted documents with explanatory historical prose while retaining the relevant design context.	Updates specification, ADR, profile, and trace README references to describe removed sketches and resolved reconciliation work without dangling links. Leaves append-only historical logs unchanged.	SPEC.md docs/adr/0004-dead-capability-surface.md docs/adr/0007-protocol-product-boundary.md docs/profiles/context-exchange-provider.md contextgraph-trace/README.md
Makes the new onboarding guide discoverable from the primary documentation entry points.	Adds an Engineer's Guide link and onboarding prompt to the repository README and documentation index.	README.md docs/index.md

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