test(#1074): agent-size-budget per-file baseline + line→byte rebase (PR 3/3) (#1097)

Extends the #1074 scheme to tests/agent-size-budget.test.cjs, which used the
same assertTightCeiling tier ratchet but was still line-based (never rebased in
#717). Completes the migration — the last part of the #1074 epic.

- Rebase agent sizing from lines to LF-normalized bytes (#717/#683).
- Delete the 'SIZE: tier anti-creep' describe (3 assertTightCeiling tests);
  add a per-agent baseline (tests/agent-size-baseline.json) as the primary
  anti-creep, and byte hard caps (XL 56 KiB / LARGE 48 KiB / DEFAULT 24 KiB),
  each above its tier high-water with real headroom. No separate new-file cap:
  a net-new agent is DEFAULT-tier, already bounded by the DEFAULT cap.
- Keep the agent-classification tests verbatim.
- scripts/workflow-size.cjs: add generic measureMdFiles(dir, predicate)
  (workflows + agents share one byte-measurement path); measureWorkflows now
  delegates to it.
- scripts/update-size-baseline.cjs: one 'npm run size:baseline' now regenerates
  BOTH the workflow and agent baselines (gsd-* filter for agents).

Rebased onto next after PR 2/3 (#1096) merged: replicate the
scripts/lib/workflow-size.cjs -> scripts/workflow-size.cjs move (PR 1/3) across
the generator and the agent test's require; regenerate the agent baseline
against current agents (a uniform +170 B preamble drift on all 33 since
authoring).

Addresses the #1097 review (trek-e):
- BLOCKER (acceptance criterion 5): document the agent contract in CONTEXT.md.
  Adds RULESET.AGENT_SIZE_BUDGET (caps 57344/49152/24576, per-agent baseline,
  dual size:baseline, shared measureMdFiles seam) and disambiguates it from the
  separate DEFECT.AGENT-FILE-SIZE-CAP-BREACH 45K-CHAR guard (two units, two
  purposes).
- Docs: now that #1096's docs/TESTING-SUITES.md "Workflow size budget" section
  is in next, fold in the agent coverage here (renamed to "Workflow & agent
  size budget"): agent caps + per-agent baseline + the how-to + reference rows,
  and the disambiguation from the 45K-char guard.
- Minor (negative proof): add a boundary-fixture test exercising the hard-cap
  comparison at cap-1/cap/cap+1 through the real lfByteCount path, so a future
  threshold/operator edit can't silently neuter a cap.
- Nit: align the tier test name wording ("stays within") with the <= operator.

Negative proof on a real tracked agent (gsd-planner): baseline catches +10 B;
XL hard cap catches 57,516 > 57,344 with the baseline current.

Closes #1095 (PR 3/3 child); landing this completes the #1074 epic.

CONTRIBUTING.md

@@ -856,9 +856,10 @@ gsd-core/
                           If you legitimately grow or shrink a workflow file,
                           run `npm run size:baseline` to update the snapshot and
                           justify any growth in your PR (or extract content
-                          lazily). Full how-to + reference in
-                          docs/TESTING-SUITES.md (Workflow size budget); see
-                          issue #1074.
+                          lazily). The same guard covers agent files
+                          (agents/gsd-*.md). Full how-to + reference in
+                          docs/TESTING-SUITES.md (Workflow & agent size
+                          budget); see issue #1074.
   references/           — Reference documentation (.md)
   templates/            — File templates
 agents/                 — Agent definitions (.md) — CANONICAL SOURCE