Propose /explore page: local user discovery directory

Stream F from docs/information-architecture.md. Closes the gap between
"I want to follow someone on this instance" and "I have a username
from outside the app."

This change is the OpenSpec proposal only — the four artifacts
(proposal.md, design.md, specs/, tasks.md) plus a new top-level spec
target at openspec/specs/explore/. Implementation lands in a follow-up
PR via /opsx:apply.

Key decisions captured in design.md:

- Anonymous access is allowed (the data shown is already public, and
  it's a stronger first-visit experience).
- Directory order: MAX(activities.created_at) DESC NULLS LAST,
  tiebreaker users.id DESC. Recency-of-activity is the simplest signal
  that meaningfully reflects "who's active here right now."
- "Active recently" sub-section: same query, sliced — top 5 users
  with a public activity in the last 30 days, hidden when empty.
- Privacy filter: profile_visibility = 'public' AND not the demo
  persona AND (forward-compat) not banned/suspended.
- No search in v1 — deferred until the user count makes it actually
  useful. /users/<username> already partially solves it.
- Offset pagination (?page=, ?perPage=, capped 100). Cursor
  pagination is the right answer at ~10K users; not now.
- Navbar gets an "Explore" entry for signed-in users; anonymous
  visitors reach /explore via a link on the visitor home.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

openspec/changes/add-explore-page/.openspec.yaml

@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-04-26