fix(cli): push-agent --help, stale --token/--max-samples help, README assets table

[harshithsunku]

Four small user-facing defects.

Changes

• perflens push-agent --help printed nothing useful. _run_push_agent took argv[0] as the ssh host unconditionally, so --help ran ssh -p 22 ... --help and failed with ssh failed: unknown option -- -. -h/--help (anywhere in the args) now print the usage to stdout and return 0. Any other argument starting with - is a usage error (exit 2) before ssh or scp run, so it can't be parsed as one of their options.
• --token help described the pre-0.10.0 model. It said agents must present a shared secret in their hello. Since 0.10.0 the server presents the pairing code to the agent through the auth command, and the agent sends no secret. The new text matches the Server CLI table in README/CLAUDE.md, and notes that a pre-0.10.0 agent's hello token is still accepted with a warning. Checked against authenticate_agent in agentlink.py (used by both inbound and outbound connections) and SECURITY.md.
• --max-samples help said 850 MB. The measured plateau at the default 500000 is near 1.1 GB, which README, docs/reference.html, CLAUDE.md and STATUS.md already say. docsContent.ts gives no memory figure, so it needed no change.
• README release-assets table. A prose line sat between rows and cut off the perflens-tools-linux-{x86_64,aarch64}.tar.gz row, which rendered as plain text. The row is back in the table, with the prose after it.
• CHANGELOG Unreleased → Fixed entry.

Test plan

• pytest tests/: 317 passed, 45 skipped (run without src/perflens/ui/, as in CI)
• ruff check src/ tests/ tools/: clean
• New tests in tests/test_cli.py mock subprocess.run and assert ssh is never invoked for --help, -h, user@host --help (exit 0, usage on stdout), and for --port, -oProxyCommand=true, user@host -p (exit 2, usage on stderr)
• The new tests fail on master: 6/6 reach subprocess.run
• Checked by hand: push-agent --help → exit 0; push-agent --bogus → exit 2; serve --help shows the new --token and --max-samples text

🤖 Generated with Claude Code