chore: rewrite onboarding docs + bump version (v0.7.3.0)

README rewritten to the install.sh + init --remote + join flow; dropped the
macOS-only prerequisite (age+jq install hints now cover apt/dnf too). CHANGELOG
entry for EGB-671.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,42 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to a four-digit MAJOR.MINOR.PATCH.MICRO version scheme.
 
+## [0.7.3.0] - 2026-06-08
+
+### Added
+
+- **Real install / onboarding scripts (EGB-671)** — onboarding a machine is now
+  (close to) one command, and a mis-copied key fails loudly instead of silently.
+  - **`secrets join --remote <url> --key <path>`** — second-machine onboarding in
+    one verb: clones the vault, installs the key at mode 600, and **verifies the
+    key actually decrypts the store before declaring success**. An empty vault
+    reports "nothing to verify yet" (it never prints a false `VERIFIED`); a wrong
+    key fails loudly with the store left in place to fix. All security logic
+    (store resolution, URL handling, path rails) is reused from the audited core,
+    not re-implemented in a side script.
+  - **`secrets init --remote <url>`** — wires the remote and pushes the initial
+    store so the upstream branch exists, so your first project `push` doesn't trip
+    the fast-forward-pull guard on a brand-new empty remote. Run interactively,
+    `init` also offers to add your first project's secrets (default No, skipped
+    under `--yes` / non-interactive, so it stays a clean primitive for CI).
+  - **`install.sh`** — thin bootstrap that ships in the repo: checks `age` + `jq`
+    + `git`, then prints the `PATH` line, the onboarding next-steps, the upgrade
+    one-liner, and a key-transfer hint. It never edits your shell config and never
+    runs `sudo` (it prints the command so you stay in control).
+  - **First-manifest `options.autoAdd` prompt (EGB-677 contract #2)** — the first
+    `push` that scaffolds a project's manifest now records an explicit, committed
+    `options.autoAdd` value (asked once when interactive; the default ON, written
+    explicitly, under automation).
+
+### Fixed
+
+- **Day-2 silent decrypt failure** — `secrets pull` now dies loudly when a blob
+  fails to decrypt with the current key (all three decrypt paths), instead of
+  emitting a warning and continuing with exit 0. A wrong key can no longer pass
+  unnoticed after onboarding.
+- The `secrets init` second-machine trap now points at `secrets join` (the real
+  one-command path) instead of a manual `git clone`.
+
 ## [0.7.2.0] - 2026-06-08
 
 ### Added