docs: add the Coven merge guide (unified CLI, user migration, Cave plan)

[BunsDev]

Summary

• Adds docs/unification.md — the user-facing guide to the Coven CLI merge: what it is, where it stands, migration steps per audience, and the comprehensive Cave (coven-cave) integration plan. Links it from COVEN.md's Engine contract section and the docs index, and adds the Phase 3 sub-plan doc.

Context

• Related issue/user request: recovered from local work on main; refreshed against tonight's landings before opening.
• Scope: docs only. The recovered draft referenced the coven PR stack #347-#349, which was superseded — the status table now reflects the re-landed and merged equivalents (coven #353/#354/#355) plus phases 4 (feat(core): unify engine home under ~/.coven with in-place migration (Phase 4, Track C) #153) and 5 (feat(brand): rebrand user-facing surfaces to Coven + direct-invocation notice (Phase 5) #154) engine-side landings, the in-place ~/.coven/code/ migration, and the still-opt-in daemonLedger default (verified against crates/core/src/lib.rs).
• Non-goals: no code changes; no changes to the plan docs' content beyond adding the phase 3 sub-plan file.

Changes

• docs/unification.md (new, ~415 lines): merge rationale (license/process boundary), phase status table, per-audience migration steps, env-var table, Cave stage-by-stage plan, compatibility guarantees.
• docs/superpowers/plans/2026-07-12-phase3-engine-harness.md (new): the Phase 3 sub-plan referenced by the guide.
• COVEN.md: one sentence in "Engine contract" linking the guide.
• docs/index.md: index entry.

Validation

• git diff --check — clean
• cargo fmt --all — N/A, docs only
• cargo check --workspace — N/A, docs only
• cargo clippy --workspace --all-targets -- -D warnings — N/A, docs only
• cargo test --workspace — N/A, docs only
• Targeted/manual checks: every PR reference in the status table verified against live PR state via the GitHub API; daemonLedger default and config_home verified in source; engine.lock location verified in the coven repo (crates/coven-cli/engine.lock).
• Not run: Rust gates — no Rust changes.

PR Readiness

• Diff is limited to the intended files.
• Generated files were not edited by hand.
• User-facing docs/help were updated or are not needed.
• No secrets, credentials, local paths, or unrelated logs are included.
• Remaining risks or follow-up work are listed above.

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Project	Deployment	Actions	Updated (UTC)
docs	Ready	Preview	Jul 14, 2026 9:53am

[Copilot]

Pull request overview

Adds user-facing documentation for the unified coven CLI + coven-code engine merge, including migration guidance and a detailed Cave integration plan, and wires the new guide into the existing docs entry points.

Changes:

• Add docs/unification.md: unified CLI merge overview, phase status, migration steps, env var reference, and Cave implementation plan.
• Add docs/superpowers/plans/2026-07-12-phase3-engine-harness.md: Phase 3 sub-plan referenced by the merge guide.
• Link the new guide from COVEN.md (Engine contract section) and docs/index.md.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
docs/unification.md	New end-to-end guide for the unified CLI merge, including migration steps and Cave integration plan.
docs/superpowers/plans/2026-07-12-phase3-engine-harness.md	New Phase 3 sub-plan document referenced by the merge guide.
docs/index.md	Adds docs index entry pointing to the new unification guide.
COVEN.md	Adds a pointer from the Engine contract section to the unification guide.

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