feat: add sshot-style playbook execution, Makefile workflows, and Cobra-style CLI help

[embh0]

Summary

Adds a lightweight, sshot-style playbook execution mode to bssh, plus Makefile-based developer workflows and a Cobra-style restructure of the CLI help output.

• Playbook execution (src/playbook/): YAML playbooks with plays, tasks, inventories, variables, fact gathering, conditions (==, !=, is defined, is not defined), register, retries, timeouts, delegate_to, run_once, sudo, and group ordering/dependencies. Includes examples/playbook.yaml and examples/playbook-inventory.yaml.
• Makefile workflows: common build/test/lint targets, a bootstrap-rustup target that installs rustup if missing, and ~/.cargo/bin prepended to PATH for all cargo targets so the Makefile works on machines without cargo on PATH.
• Cobra-style CLI help: compact grouped -h output (flags grouped under Target Selection, Configuration, Authentication, Connection, Port Forwarding, Output, Execution Control, Terminal), an "Available Commands" section, full examples moved to --help via long_help/after_long_help, and a per-command help footer — mirroring the help layout of Go's Cobra library.
• Clippy: needless_borrows_for_generic_args fixes in the new playbook code.

Rebased onto current main (post-v2.3.1). cargo clippy --workspace --all-targets -- -D warnings is clean on both the pinned toolchain and current stable.

Test plan

• cargo test --lib (full suite passed on the pre-rebase branch; 1247 passed, 0 failed)
• Manual verification of bssh -h, bssh --help, and bssh <cmd> -h output
• Playbook unit tests included under src/playbook/ (condition evaluation, template rendering, parser)

🤖 Generated with Claude Code

https://claude.ai/code/session_015dJq1xmuHgigcB14VCfa17

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