feat(error): let DetailedError contribute structured next_actions to error envelopes

[mguerrero3-godaddy]

Summary

• Adds DetailedError::error_next_actions() -> Vec (default-empty, mirroring error_fix()) so error envelopes can carry structured, agent-executable next actions instead of only a plain-text fix string.
• CliCoreError::Detailed now captures these eagerly at wrap time (before the source is erased to dyn Error), and both build_error_envelope and build_detailed_error_envelope thread the value through instead of hardcoding next_actions: Vec::new().

Implements for ticket DEVEX-983

Test plan

• cargo fmt --all --check
• cargo clippy --all-targets -- -D warnings
• cargo test --all-targets

Manual verification

Setup:

# In the cli repo, temporarily override the engine dependency:
cd cli/rust
# Edit Cargo.toml → cli-engine = { features = ["pkce-auth"], path = "../../cli-engine" }
cargo build --release && cp target/release/gddy ~/.local/bin/gddy

Test WITHOUT the fix (baseline):

cd cli-engine && git checkout main
cd cli/rust && cargo build --release && cp target/release/gddy ~/.local/bin/gddy
# <command to reproduce the issue>
# Expected: <describe broken behavior>

Test WITH the fix:

cd cli-engine && git checkout <this-branch>
cd cli/rust && cargo build --release && cp target/release/gddy ~/.local/bin/gddy
# <same command>
# Expected: <describe fixed behavior>

Cleanup:

# Revert cli/rust/Cargo.toml back to:
# cli-engine = { features = ["pkce-auth"], version = "<published-version>" }