feat: journal access notice, 1.60.0, install script, changelog, riscv protoc fallback

- Journal pane now distinguishes 'no entries' from 'no journal access':
  journalctl exits 0 with empty output when the agent's user simply can't
  see the target's entries (demo mode / user-run agents), explaining
  itself only on stderr. The agent forwards that hint as an additive
  JournalResponse.notice and the client renders it with practical advice.
  Verified E2E via a stub journalctl emulating the unprivileged case.
- Version 1.60.0 across all crates (1.51 would read fine, but the repo's
  scheme is 1.40/1.50/…, and a literal 1.6.0 would sort BELOW 1.50.0 in
  semver). All user-facing version strings already come from
  CARGO_PKG_VERSION — a stale binary was the only way to see an old one.
- scripts/install.sh: build-from-source install/upgrade for the test
  fleet (Linux + macOS). Detects in-repo checkouts, installs rustup when
  missing, replaces a systemd socktop-agent service binary in place and
  restarts it, requires system protoc on riscv64.
- build.rs (agent + connector): fall back to $PROTOC / PATH when
  protoc-bin-vendored has no binary for the host (riscv64) — native SBC
  builds previously panicked in the build script.
- CHANGELOG.md covering v1.50.0 -> 1.60.0.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Cargo.lock

@@ -2412,7 +2412,7 @@ dependencies = [
 
 [[package]]
 name = "socktop"
-version = "1.51.0"
+version = "1.60.0"
 dependencies = [
  "anyhow",
  "assert_cmd",
@@ -2431,7 +2431,7 @@ dependencies = [
 
 [[package]]
 name = "socktop_agent"
-version = "1.51.0"
+version = "1.60.0"
 dependencies = [
  "anyhow",
  "assert_cmd",
@@ -2462,7 +2462,7 @@ dependencies = [
 
 [[package]]
 name = "socktop_connector"
-version = "1.51.0"
+version = "1.60.0"
 dependencies = [
  "flate2",
  "futures-util",