Add shell autocomplete (bash/zsh) for aegisctl #81

Problem

aegisctl has a growing list of subcommands (pending, approve, deny, verify, evidence, simulate, why, diff-policy, manifest, supply-chain, policy-pack, test-action, plugin). Typing them without tab completion is a drag.

Acceptance criteria

• New command: aegisctl completion bash prints a bash completion script to stdout
• New command: aegisctl completion zsh prints a zsh completion script to stdout
• Completion covers top-level subcommands (not every nested flag, that would be a lot)
• Short instructions for installing it live in starter-kit/QUICKSTART_PR_WRITER.md or a new docs/aegisctl-completion.md
• Unit test verifies the output contains the expected subcommand names

Files to touch

• cmd/aegisctl/completion.go (new)
• cmd/aegisctl/main.go (wire the new subcommand)
• cmd/aegisctl/completion_test.go (new)
• Optional: docs/aegisctl-completion.md for installation instructions

How to test locally

./bin/aegisctl completion bash > /tmp/aegisctl-completion.bash
source /tmp/aegisctl-completion.bash
# Now type "aegisctl " and press TAB

Notes

No need to use a fancy library for this. A hand-written bash function is fine for a project this size. Look at how gh, kubectl, or docker handle their completion scripts for inspiration.