md-cli v0.6.0 — md repair subcommand

Tranche B Phase B.6 of the v0.22.x follow-ups cycle. Sibling-CLI repair surface mirroring toolkit's `mnemonic repair --md1`.

What's new

• `md repair ...` — variadic positional multi-chunk repair via md-codec v0.34.0's `decode_with_correction`. D28 atomic semantics: if ANY chunk fails BCH capacity → whole call fails with `chunk_index` named, NO partial output emitted.
• Exit codes per D26: `0` (clean) / `5` (REPAIR_APPLIED) / `2` (unrepairable).
• `--json` envelope byte-matches toolkit's `RepairJson` schema per D27 (`schema_version="1"`, `kind="md1"`, `corrected_chunks`, `repairs[].{chunk_index, original_chunk, corrected_chunk, corrected_positions[].{position, was, now}}`).
• D25 handler-signature cascade — 9 existing handlers refactored to `Result` for exit-5 propagation.

Closes FOLLOWUPs

• `md-cli-repair-flag` (toolkit + this repo)

Lockstep

• mnemonic-toolkit v0.23.0 — toolkit's `mnemonic repair --md1` shares the JSON envelope schema; sibling CLIs are parser-reuse compatible.
• ms-cli v0.4.0 — sibling `ms repair`.
• mk-cli v0.4.0 — sibling `mk repair` (released earlier in cycle).

🤖 Released as part of the v0.22.x follow-ups cycle (Tranche B close).