[issue-3288][slice-3/6] Total-refactor load-bearing architecture pages

[james-in-a-box]

Rewrite the architecture pages whose ledger framing is load-bearing into current-state prose (not a line edit); preserve genuinely useful rationale, reframed and isolated where substantial. Highest judgment cost — isolated into its own slice.

Base PR: #3317

What's in this PR

Commits (3):

• Persist BRC history for slice-3 (Slice PRs are missing analysis/plan docs and all BRC history; need a 'context' PR for refine+plan phases plus per-slice BRC in each slice PR #2548)
• docs(slice-dag): strip v6 tag and title hard-cut change-log narration
• implement(Documenter agent + docs: snapshot of current state, not a ledger of changes #3288): de-ledger slice-3 architecture pages

.egg-state/brc-history/3288-implement-slice-3.json | 487 ++++++++++++++++++++++++++++++++++++
 .egg-state/brc-history/3288-implement-slice-3.md   | 543 +++++++++++++++++++++++++++++++++++++++++
 docs/architecture/coordination-state.md            |  91 +++----
 docs/architecture/gateway-auto-filter.md           | 245 ++++++++++++-------
 docs/architecture/slice-dag.md                     | 312 +++++++++++------------
 5 files changed, 1363 insertions(+), 315 deletions(-)

This slice

Total-refactor load-bearing architecture pages

Files affected:

• docs/architecture/gateway-auto-filter.md
• docs/architecture/coordination-state.md
• docs/architecture/slice-dag.md

Tasks (3) + acceptance criteria

• task-3-1: Rewrite docs/architecture/gateway-auto-filter.md: remove the "## What was removed" inventory and the "preserved for context" description of the removed historical Gateway should auto-filter disallowed files on push, and handle pulled cross-role commits #1882 design — keep only what a reader of the current filter needs (a one-line rationale link is acceptable).
  • Acceptance criteria: - gateway-auto-filter.md has no "what was removed" / "preserved for context" historical-record section; current behavior stands alone.
• task-3-2: Rewrite docs/architecture/coordination-state.md: replace "This page describes the final shape of the Design: coordination state is served, not replicated — retire git/prompt choreography for agent state exchange #3077 invariant. The epic was landed in six slices, all of which have now shipped…" with a direct statement of the current invariant, and convert the Threat/Mitigation/Status tables full of "Retired surface" / "now historical" rows into the current threat model (drop retired rows or fold them into a short rationale note).
  • Acceptance criteria: - coordination-state.md states the current invariant directly with no "landed in six slices" chronology; status tables carry only live rows (retired rows removed or reduced to rationale).
• task-3-3: Rewrite docs/architecture/slice-dag.md: replace "The implement phase used to run as a single monolithic agent team… The slice-DAG model replaces that…" with a description of the current slice-DAG model. A brief rationale note on why the model exists is fine; the chronology is not. The word "slice" is a live domain concept here — keep it; strip only the change-log narration and slice-NUMBER ledger references.
  • Acceptance criteria: - slice-dag.md describes the current model; "used to… replaces that…" narration is gone; the live "slice" domain term is retained.

Stack

• Position: slice 3 of 6 in pipeline issue-3288
• Base PR: Documenter snapshot-not-ledger framing + corpus cleanup (#3288) #3317
• Stacked on top of egg/issue-3288/slice-1