feat(unlock): Read password via stdin

[augustebaum]

Caution

This code is provided with no security guarantees. I'm writing this for learning purposes and because it seems there is demand for this feature.

Introduces parameter --stdin to rbw unlock; when set, the password will be read from the first line of standard input. The --stdin parameter is inspired from jj describe.

Tested manually with

# Need to be running the agent with this PR's changes
rbw stop-agent
cargo run --bin rbw-agent

echo password | cargo run --bin rbw -- unlock --stdin
# pinentry does not show up
# rbw unlock: failed to unlock database: Password is incorrect. Try again.

I'm not quite sure how to test this feature automatically.

Closes #65

AI disclosure statement

I used Claude Code to think about the feature, and in particular about the security aspects (e.g. zeroing memory aggressively). I wrote and edited the code myself; I feel comfortable calling this my work.