Each of the four READMEs was trying to do too many jobs (pitch, status
bulletin, reference, architecture, examples, changelog), which made
the landing pages stale, bloated, and internally inconsistent with the
three-crate product story that #95 landed. This rewrite aligns them
around one job per page.

Role allocation after this PR:

- README.md (root) — ROUTE. Problem statement, one quick-start, one
  "what Shipper doesn't do" boundary, links into Diátaxis docs.
  No release-history prose, no runtime metrics, no audience matrix,
  no config/CI walkthrough — those live in their own docs.

- crates/shipper/README.md — INSTALL FACE. What users see on
  crates.io for the install target. Problem, install, four-command
  quick-start, pointer to the sibling crates for lean embedding.

- crates/shipper-core/README.md — SDK PAGE. Grew from a 25-line
  positioning note to a proper embedder onboarding: when to pick it,
  what lives here, what doesn't, the API shape to expect, and a
  minimal code sketch.

- crates/shipper-cli/README.md — ADAPTER PAGE. Deliberately the
  shortest. Tells users to install `shipper` first, shows when to
  reach for the adapter directly (custom wrappers, explicit install
  of `shipper-cli`), points lean embedders at `shipper-core`.

Cut stable across all four: dated release-history prose, 41-retries /
69-minute run metrics, audience matrix, full command catalogs, long
architecture narration. Those belong in CHANGELOG.md, roadmap, and
the Diátaxis docs tree — not on landing pages.

Totals: 497 → 219 lines (≈56% reduction) across the four files,
while `shipper-core` actually grew because it was too thin before.

Verified:
- cargo check --workspace clean
- grep for "compatibility shim" / rc.1 release prose / stale install
  commands returns no hits in the rewritten files.

Follow-on: docs/release-runbook.md and docs/product.md still carry
stale rc.1 references — out of scope for this PR; will follow in
separate doc-cleanup work.

…lity

Follow-on from the four-README rewrite. Two agents did the heavy
lifting; this commit lands their work.

docs/product.md:
- Dropped the stale `What's shipped (v0.3.0-rc.1)` capability-status
  table (rows were either outdated or duplicated ROADMAP content).
- Added a three-crate Product Shape section matching the new READMEs
  (shipper = install face, shipper-cli = adapter, shipper-core = engine).
- Added a concise What Shipper Does Today section naming each of the
  nine competencies as present capabilities, pointing at ROADMAP for
  status and CHANGELOG for per-release detail instead of duplicating.
- Expanded Compared to alternatives to include cargo publish --workspace
  (Cargo 1.90+).
- Rewrote the Where to go from here table as role-based navigation,
  including an embedder row pointing at shipper-core.
- Kept the identity sentence, the "intentionally narrow" framing, and
  the audience paragraph pattern — those were already steering-shaped.

docs/release-runbook.md:
- Dropped the `Current RC baseline: v0.3.0-rc.1 / 5beab9b` framing and
  the hardcoded `git tag -a v0.3.0-rc.1` example.
- Replaced with a VERSION=$(cargo metadata...) pattern so operators
  tag from Cargo.toml rather than hardcoding per-release.
- Updated crates-in-the-train to 13 with the new tier structure
  (tier 4 core → tier 5 cli → tier 6 install façade).
- Reframed auth as "token fallback (primary today) / Trusted Publishing
  (target path)", matching what PR #150 actually landed and the live
  state of the crates.io-side TP registration.
- Rewrote walk-back section to lead with `shipper plan-yank` /
  `shipper fix-forward`, manual `cargo yank` as fallback.
- Deliberately kept the single link to release-v0.3.0-rc.1-manifest.md
  and the "as of rc.2" capability signal — both are intentionally
  version-pinned historical markers.

Totals: docs/product.md 530 words, docs/release-runbook.md ~250 lines
of version-agnostic procedure the operator can re-run for rc.3,
0.3.0 stable, 0.4.0, etc. without edits.