Final step of the #95 split. After PRs 1-3 landed the code in the right
crates, repo-level docs still described a two-crate (`shipper` +
`shipper-cli`) layout and pointed install commands at
`cargo install shipper-cli`. This PR brings the docs in line with what's
actually on main.

## Updates

- **`README.md`** — workspace crates section now shows the three-crate
  product surface: `shipper` (install), `shipper-cli` (CLI adapter),
  `shipper-core` (engine).
- **`CLAUDE.md`** — replaces the two-crate description with the layered
  three-crate architecture, updates the build/test commands to target
  the right crates (`-p shipper-core` for engine work,
  `-p shipper-cli` for CLI work, `-p shipper` for façade tests), and
  explains where behavior / CLI / surface changes belong.
- **`GEMINI.md`** — same architectural rewrite; project-structure
  section rewritten to match the new crate layout.
- **`.github/copilot-instructions.md`** — rewritten to describe the
  three-crate shape, where work goes, and which paths to use in docs
  (`shipper_core::...` for engine internals, not `shipper::...`).
- **`docs/structure.md`** — adds the three-crate diagram, replaces the
  `crates/shipper` module map with a `crates/shipper-core` map, and
  adds module maps for `shipper-cli` and the `shipper` façade.
- **`docs/architecture.md`** — module path references (`shipper::engine`,
  `shipper::ops::auth`, `shipper::state::events`, etc.) rewritten to
  `shipper_core::...` so readers aren't chasing paths that no longer
  resolve from outside the façade.
- **`docs/how-to/run-in-github-actions.md`** + **`templates/*.yml`** —
  `cargo install shipper-cli --locked` → `cargo install shipper
  --locked`. The old command still works (shipper-cli ships a compat
  binary), but the docs should push operators toward the product name.
- **`crates/shipper-cli/README.md`** — rewritten to describe this crate
  as the CLI adapter (not "the CLI frontend over the shipper library"),
  with clear guidance on when to depend on `shipper-cli` vs
  `shipper-core` vs just installing `shipper`.

## What this PR does not do

- Does not physically move the integration tests in `crates/shipper/tests/`
  into `crates/shipper-core/tests/`. They're still valid (they exercise
  the curated re-export surface) and moving them would double the churn
  in this PR for little gain. Flagged for a future cleanup if someone
  wants.
- Does not touch `docs/decrating-plan.md` — that doc is historical and
  describes the absorption phase. Its `shipper::...` references are
  correct-at-the-time narrative, not operator-facing guidance.
- Does not touch `RELEASE_NOTES_v0.2.0.md` (historical).

## Verification

- `cargo check --workspace --all-targets` — clean
- `cargo clippy --workspace --all-targets --all-features -- -D warnings` — clean

Closes #95.