feat(cli): add --raw and --base64 flags for clean SSH key import/export

[f3rdy]

Summary

SSH private keys and certificates stored in Ansible Vault suffer from whitespace corruption during YAML multiline formatting. This PR adds clean import/export modes to prevent these issues.

• get --raw: Outputs value without Type: headers or field labels, strips trailing whitespace per line, ensures single trailing newline. Ideal for vaultctl get key --field privateKey --raw > key.pem
• get --base64: Outputs value as a single base64-encoded line, suitable for environments that cannot handle multiline values
• set --base64: Accepts an inline base64-encoded value, decodes before storing
• set --base64-file: Reads base64 from a file or stdin (-), decodes before storing
• set --file: Now applies whitespace cleanup (trailing space removal) on import
• clean_multiline_value(): New helper that strips trailing whitespace per line and ensures exactly one trailing newline

Problem

When SSH keys are stored in YAML via ansible-vault, the multiline formatting introduces trailing spaces on lines. Extracting these keys with vaultctl get ... --json | jq -r produces keys that SSH rejects. There was no way to get a clean, whitespace-safe export or to import base64-encoded values.

Changed Files

File	Change
src/vaultctl/cli.py	Added --raw and --base64 flags to get, --base64 and --base64-file options to set, mutual exclusivity validation, _output_raw() and _output_base64_encoded() helpers
src/vaultctl/yaml_util.py	Added clean_multiline_value() helper
tests/test_cli.py	17 new integration tests covering all new flags and edge cases
tests/test_yaml_util.py	7 unit tests for clean_multiline_value()
tests/conftest.py	Added ssh_key fixture entry with trailing whitespace for testing

Design Decisions

1. clean_multiline_value in yaml_util.py — It is a value formatting utility closely related to YAML handling, keeping it here avoids a new module for one function
2. Mutual exclusivity of --json, --raw, --base64 — Validated at runtime with a clear error message rather than Click's cls=MutuallyExclusiveOption to keep it simple
3. --file now cleans whitespace on import — Prevents storing corrupted values at the source. This is a minor behavioral change but strictly an improvement
4. --base64-file - for stdin — Follows Unix convention, enables piping: cat key.pem | base64 | vaultctl set key --base64-file -

Test Plan

• get --raw on plain strings outputs clean value
• get --raw on multiline values strips trailing whitespace
• get --raw --field extracts single field without headers
• get --raw on structured entries outputs YAML without Type: header
• get --base64 produces valid single-line base64
• get --base64 on multiline values cleans before encoding
• get --base64 --field works on individual fields
• --json, --raw, --base64 are mutually exclusive
• set --base64 decodes and stores correctly
• set --base64 rejects invalid input
• set --base64-file reads from file
• set --base64-file - reads from stdin
• set --file cleans trailing whitespace
• Multiple input sources rejected
• clean_multiline_value unit tests (7 cases)
• All 319 tests pass, coverage 88%