docs(W4): stamp kind and status front matter on 104 live documents

Applies the document contract from
docs/plans/2026-08-20_stack-docs-flatten-and-alignment.md section 3, partially:
`kind` and `status` only. `parent` is deliberately held until the flatten in
section 4 lands, so that 127 documents do not have to be re-pointed by hand
when docs/fleet/NORTH_STAR.yaml moves to docs/NORTH_STAR.yaml.

Scope, measured on origin/next at 63069149:

  127 live docs   = all *.md under docs/ minus docs/archive/ minus docs/_old_structure/
  104 stamped     here
   19 held        operator judgement (plan section 9), worklist in the same PR
    3 held        the SUPERSEDED TASKS.md stamps, which cite the moving path
    1 untouched   docs/fleet/FLEET-DOCTRINE.md, already stamped in W1

Kinds applied: 54 guide, 34 record, 9 spec, 6 tracking, 1 projection.
Every row carries a confidence and a one-line rationale in the worklist.

Two collisions with the existing state, both flagged rather than resolved:

1. docs/README.md:150-160 already documents a front-matter convention
   (title/type/audience/status/source_of_truth) with its own allowed values.
   It is applied to 4 of 127 files. Its `status` vocabulary is
   current|draft|deprecated|historical; the new contract's is active|superseded-by.
   The key collides. This commit lets the new contract win and rewrites
   `status: current` to `status: active` on those 4 files, keeping their other
   legacy keys untouched. No code reads any of them: `git grep source_of_truth`
   outside docs/ returns nothing. docs/README.md still prescribes the old
   convention and is an operator row, so it is not edited here.

2. Two of the plan's 20 operator rows are YAML files, not markdown
   (docs/fleet/examples/roster-v2.yaml, docs/openapi-tess.yaml), and the
   contract's front-matter form has no defined meaning for a .yaml document.
   That gap also applies to docs/fleet/NORTH_STAR.yaml, the source of truth
   itself. Raised in the worklist.

A third row from the plan, docs/fleet/north-star.md, no longer exists: W1
renamed it to docs/fleet/FLEET-DOCTRINE.md.

Verification: 104/104 parse with the expected kind and status in front matter;
the check was shown to reject a wrong kind before it was trusted. The diff
removes 4 lines total, all of them `status: current`.

docs/fleet/PRD.md

@@ -1,3 +1,8 @@
+---
+kind: spec
+status: active
+---
+
 # PRD — Fleet Phase 2: Operator Observability
 
 > **Workstream:** W-FLEET under `mvp-20260312` · **Phase:** 2