feat(cli): add dynamic shell completions and ValueHint annotations

[joshrotenberg]

Summary

• Enable runtime-aware tab completions via clap_complete's CompleteEnv, adding profile name completion from the config file, file/URL/executable path hints for relevant args
• Add --register flag to redisctl completions <shell> that prints the shell one-liner for dynamic completion setup
• Static script generation (redisctl completions <shell>) preserved for backward compatibility

Changes

File	What
crates/redisctl/Cargo.toml	Enable unstable-dynamic feature on clap_complete
crates/redisctl/src/cli/mod.rs	ValueHint annotations (FilePath, Url, ExecutablePath), ArgValueCandidates on --profile, --register flag, profile_candidates() completer
crates/redisctl/src/main.rs	CompleteEnv at top of main(), print_registration_command(), updated handler and format_command
docs/docs/reference/shell-completions.md	"Dynamic Completions (Recommended)" section, static completions moved to fallback

Test plan

• cargo fmt --all -- --check
• cargo clippy -p redisctl --all-targets --all-features -- -D warnings
• cargo test -p redisctl --lib --all-features (72 tests)
• cargo test -p redisctl --bin redisctl --all-features (105 tests)
• redisctl completions bash -- static script still generated
• redisctl completions bash --register -- prints source <(COMPLETE=bash redisctl)
• Add registration line to shell rc, verify profile names complete with --profile <Tab>

Closes #750