[CLI] Decide compatibility policy for unified options

[nth-bailey]

Context

#50 proposes hidden aliases and migration warnings for existing CLI flags. PR #56 introduces --backend, --style, and repeatable --feature, but removes the old --zod, --source-gen, and --rkyv flags instead of retaining aliases; --zero-copy remains a first-class option because it also supports an explicit false value. This is a user-visible compatibility decision, so #50 must remain open after PR #56 merges.

Investigate and resolve

• Check which legacy flags and manifest keys were documented and used by released versions, and whether a breaking removal is appropriate for the next release.
• Decide which legacy flags and keys need temporary aliases, the warning text and removal version, and how --zero-copy=false maps to the unified options.
• Implement the chosen behavior in CLI parsing and manifest loading, with tests for successful migration, warnings, conflicts, and invalid target combinations.
• Update migration notes and CLI examples. Record the decision in [CLI & Architecture] Consolidate target-specific boolean flags into unified options (--backend, --style, --feature) #50.

Related: #50, PR #56.

[nth-bailey]

Decision: remove the old one-off flags and manifest keys directly. We will not add compatibility aliases, deprecation notices, or a migration guide at this stage. PR #56 already implements and tests the removal; --zero-copy remains first-class because it supports an explicit false value. #50 has been updated to reflect this policy. PR #59 corrects the remaining wording in the compiler guide and runbook.