docs(decompose): batch task creation via bd create --graph + stale gate ref cleanup (hew-qq28)

[droidnoob]

docs(decompose): batch task creation via bd create --graph

Closes hew-qq28 by reframing it as a documentation gap rather than a code gap. The original task spec proposed building hew task new-batch <manifest> to sidestep GOTCHA:zsh-cmd-substitution. While exploring the implementation I discovered bd already has the primitive: bd create --graph plan.json accepts JSON with nodes + edges + parent_key, runs as one transaction, and passes multi-line descriptions through verbatim — including apostrophes, quotes, backticks, and $() substitutions.

Reimplementing it in hew would be pure rewrap with no value-add. So instead this PR:

Documents the batch path in the skill body

skills/core/hew-decompose.md gains a new "Batch mode" section right before Step 4 (dep wiring). The example shows the actual schema (nodes[].key, parent_key, edges[].from_key/to_key/type: "blocks") with a description that includes the actual zsh-hostile shape:

{
  "description": "**What:** validates body, returns AuthResponse.\nApostrophes', \"quotes\", $(this) and `backticks` all pass through verbatim."
}

Smoke-verified: created a task with exactly that description via bd create --graph, content survived bit-for-bit.

Updates the GOTCHA highlight

CLAUDE.md's GOTCHA:zsh-cmd-substitution entry now points at bd create --graph as the recommended workaround for >3-task batches. The old bd q + bd update --body-file advice is preserved for one-off updates.

Fixes two stale references in the same skill

The decompose skill had the same ghost primitive /hew:ship had (resolved by PR #51):

• Step 5 (external gates): bd gate create --type=gh:pr ... rewritten to use hew gate new --gh-pr=N (which actually exists).
• Step 6 (type table): gate dropped from the --type= enum since bd doesn't accept it. External blockers go through the dedicated hew gate surface.

What didn't ship

No hew task new-batch subcommand, no hew_core::task_manifest module. The acceptance criteria from hew-qq28 (transactional creation, multi-line descriptions, intra-batch deps) are all met by bd create --graph already. Treating bd create --graph as a documented hold-out — like bd orphans and bd lint — keeps the surface area smaller and avoids drift.

Quality

• 924 tests pass (no code changes)
• cargo clippy --all-targets -- -D warnings clean
• cargo fmt --check clean

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