NOW.md is a single-file merge bottleneck: every open PR is CONFLICTING, and merge=union makes it worse

[gHashTag]

Problem

Every PR must update docs/NOW.md, and entries are prepended — so every PR
rewrites the same first line. All 18 currently-open PRs are marked
CONFLICTING by GitHub. The conflict is structural, not incidental: one file,
one anchor, ~35 PRs/day.

docs/NOW.md merge=union was added to make those merges automatic. Measured
against master @ 7e8de87b1, it does not do the job:

• GitHub never applies it. git merge-tree run from a worktree checked out
  at origin/master (so master's .gitattributes is in force) reports
  docs/NOW.md clean for PRs that GitHub simultaneously labels
  CONFLICTING — including tri triage: five ordered classes, and blocked is tested before actionable (#2156) #2157 and chore: civilian mesh positioning — drop drone wording (bpsk.t27) #1874, which conflict on nothing else.
  Mergeability on the platform ignores merge drivers.
• Off the platform it corrupts silently. Union's failure mode is
  duplication, not removal. Two branches editing one Last updated: line
  merge with no conflict into two adjacent Last updated: lines under a
  single heading. Under the default driver that is a conflict a human resolves.

The damage is measurable on master today:

measurement	value
^# NOW headings in docs/NOW.md	137
Last updated: lines	136
orphaned entry	Wave Loop 421 close-out / Wave Loop 422 setup (2026-07-06)
headings with union's blank-line-eaten signature	5, all dated 2026-08-19/20

So the rule buys nothing where conflicts are reported, and costs real
conflict-detection where they are not.

Two further defects found in the same sweep

1. tri hooks now-gate is dead code. cli/tri/src/hooks.rs matches
   ^\*\*Last updated:\*\* — a bold label. cli/tri/src/nownote.rs has
   only ever written the plain Last updated: form. Master contains 0
   bold occurrences against 136 plain. The gate can never pass on a real
   checkout.
2. .gitattributes describes a file that does not exist. Its comment says
   "The root file is a symlink." git ls-tree shows 100644 blob for root
   NOW.md — a divergent regular file stamped Last updated: 2026-08-09.

Proposed fix

One file per entry: docs/now/<YYYY-MM-DD>-<slug>.md. Two PRs write two
different paths, so there is no shared line and nothing for GitHub to call
dirty. This is already the repo's dominant convention — docs/reports/ holds
1,564 date-stamped files, .claude/plans/ 417, and .trinity/experience/ is
one append-only file per track.

The gate keeps asserting both halves, unweakened:

• presence — the diff must add (--diff-filter=A) a matching entry;
• freshness — that entry's filename date must fall in
  [yesterday .. tomorrow] UTC, the same window as today;
• content (new) — at least one heading and one bullet, because an empty new
  file would otherwise be the same vacuous pass a whitespace touch used to be.

merge=union is retired for both NOW.md and docs/NOW.md.

Explicitly out of scope

• The 137 existing entries are not migrated. docs/NOW.md is frozen as a
  historical archive with a pointer header. Splitting it is mechanical and would
  make the change unreviewable.
• The orphaned entry is not repaired. Its date cannot be recovered without
  guessing.
• The 18 open PRs are not touched. Each needs one rebase to move its entry
  to docs/now/; there is no migration that spares them.
• .trinity/experience.md, .trinity/current-issue.md and
  bootstrap/stage0/FROZEN_HASH have the identical pathology and are left
  alone. FROZEN_HASH is a one-line file every PR rewrites.

Note for the owner

This changes the contract of a required status check repo-wide. The PR will
be left open rather than auto-merged.