feat(P02): install.sh 1-liner + in-place update + README quickstart

REQ-043: install.sh pulls release binary from public Gitea URL.
  User-level default (~/.local/bin/orca), --system for system-level
  (/usr/local/bin/orca). Defaults to latest release; --version pins.
  Env-overridable GITEA_URL/OWNER/REPO for testability.

REQ-044: in-place update detects existing binary, reads version via
  'orca version --json', prints update message, overwrites binary,
  preserves namespace dir (config/db/certs). Idempotent re-install.

REQ-016 (completion): README quickstart now documents the 1-liner
  install + --system variant + update-in-place pattern.

Tests: 8/8 pass in scripts/install_test.sh (real public Gitea releases,
  no mock server; timeout-guarded to prevent hangs).

Docs: docs/install.md covers user/system install, version pinning,
  in-place update, uninstall, troubleshooting.

---ci---
project: orca
phase: 2
milestone: v0.5
status: verify
---/ci---

README.md

@@ -18,16 +18,43 @@ Offline/CLI-first orchestration engine inspired by HashiCorp Nomad, far simpler
 
 ## Quickstart
 
+### Install (1-liner)
+
 ```bash
-# Build
-make build
+# User-level install (binary at ~/.local/bin/orca, state at ~/.orca)
+curl -fsSL https://git.cloudinit.dev/coreci/orca/raw/branch/main/scripts/install.sh | bash
 
-# Run
-./bin/orca version
-./bin/orca --help
+# System-level install (binary at /usr/local/bin/orca, state at /root/.orca)
+curl -fsSL https://git.cloudinit.dev/coreci/orca/raw/branch/main/scripts/install.sh | sudo bash -s -- --system
 
-# Initialize local state
-./bin/orca init
+# Pin a specific version
+curl -fsSL https://git.cloudinit.dev/coreci/orca/raw/branch/main/scripts/install.sh | bash -s -- --version v0.4.2
+```
+
+Then initialize local state and verify:
+
+```bash
+orca init          # creates ~/.orca/ (or /root/.orca with --system)
+orca version       # prints version info
+orca --help        # show all subcommands
+```
+
+### Build from source
+
+```bash
+make build         # Build binary to ./bin/orca
+./bin/orca init    # Initialize local state
+./bin/orca version # Verify
+```
+
+### Update in place
+
+Re-running the installer updates the binary while preserving your
+config, database, and certificates in the namespace dir:
+
+```bash
+curl -fsSL https://git.cloudinit.dev/coreci/orca/raw/branch/main/scripts/install.sh | bash
+# → "updated orca from v0.4.1 to v0.4.2"
 ```
 
 ## Subcommands