fix(store): source_issue "none" clears it (move a split's closing edge)

[mabry1985]

There was no way to clear a card's source_issue. An empty value means "leave it", and "none" failed validation. So when a split gains a new last slice, the old last slice keeps Fixes #N and closes the issue early. Live case: the ADR 0115 split gained a docs card (bd-negi) after C6 (bd-83sh), and C6 still carried source_issue=#3760.

"none" / "clear" / "-" now clears it, the same convention as waits_for. It's in store.update_feature, so both board_update_feature and PATCH /features/{fid} get it. Files and the requirement ledger in the shared notes field are carried forward. The docstrings say how to clear.

Test: parametrized clear, keeping files_to_modify. Full suite: 2649 passed, 36 skipped. ruff clean.

🤖 Generated with Claude Code

[protoreview]

QA panel review — PASS

code-review-structural · head 78429b3712e7 · formal

Low-risk, well-scoped change: adds sentinel values ("none", "clear", "-") to source_issue in update_feature to clear the stored source-issue line, with docstring updates and a parametrized test. All four LLM finders confirm the logic is sound and consistent across call sites. The protoPatch engine produced 16 findings, all targeting pre-existing code outside this diff's scope — dropped. The single surviving finding is a naming nit on the new set_source variable, confirmed by the verifier. No panel disagreements; no coverage gaps.

Findings

	Severity	Location	Finding	Verified
⚪	nit	store.py:2229	The local variable set_source now evaluates True when the source issue is being CLEARED, so its name no longer matches its meaning.	confirmed

findings JSON (machine-readable)

[
  {
    "file": "store.py",
    "line": 2229,
    "severity": "nit",
    "category": "conventions",
    "claim": "The local variable `set_source` now evaluates True when the source issue is being CLEARED, so its name no longer matches its meaning.",
    "evidence": "set_source = (source_issue is not None and str(source_issue).strip()) or clear_source",
    "verdict": "confirmed",
    "note": "Code verified verbatim at head SHA; when clear_source is True, set_source is True, so the name misleads \u2014 a name like `touch_source` would be more accurate."
  }
]

[protoreview]

Promoting the PASS verdict for head 78429b3712e7: all checks terminal-green, zero unresolved review threads. (approve-on-green)

Open findings carried by this approval — non-blocking, but they did not go away:

• nit store.py:2229 — The local variable set_source now evaluates True when the source issue is being CLEARED, so its name no longer matches its meaning.

Approving a WARN does not resolve its findings (issue #22).