docs(adr): repair the decisions index — layout, and one row that described the rejected option

[h1d3mun3]

Two independent fixes to docs/decisions/README.md, kept as separate commits so either can be
dropped. Neither depends on the egress audit.

1. The index table was split into four

A blank line inside a GitHub-flavoured Markdown table ends it. There were three — after 0012,
after 0014 and after 0015 — so the index rendered as four separate tables, each re-drawing the
ADR | Decision header, with 0015 and 0016 as one-row tables of their own.

The rows themselves were fine; only the separators were wrong. Removing the three blank lines is the
entire fix.

Verified rather than assumed: all 16 ADR files have an index row, and all 16 rows point at a file
that exists.

2. The 0012 row described the approach 0012 rejects

The index said cmd_up_macos "writes the now-generic onboarding stub only when the file is
absent". That is not what it does — and it is not a slip, it is exactly the alternative ADR-0012
exists to rule out:

implementation	augur:3931 — if $_fresh_clone; then
its own comment	augur:3921 — "WRITE-ONCE, gated on _fresh_clone … NOT on the file's absence"
ADR-0012 §85	"only when this up just cloned the VM"
ADR-0012 §93	an existence check is "a weaker check masquerading as the same thing"

The distinction is load-bearing: the base VM is a long-lived mutable disk that can accumulate its
own stale ~/.claude.json between builds, so an existence check finds a file on a fresh clone and
silently inherits it into every new project — which is the bug 0012 was written about.

An index that states the rejected option as the implemented one sends a reader hunting for a defect
that is not there, or — worse — invites someone to "simplify" the gate back into the defect. The row
now matches the code and carries the reason.

Scope

docs/decisions/README.md only. No code, no invariant, no snapshot. Found while auditing the
decisions index during the #124/#135 series; the layout half was already on the known-issues list.

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