docs: restructure into topic subdirectories, collapse duplicated index

Per mara's go-ahead on hyperhive#3902 ("getting started is good, but
terminal rendering does not go in there i think"):

Moved 21 top-level docs/*.md files into 7 new topic subdirectories
(existing web-ui/, turn-loop/, swarm/, tools/, crates/ untouched):
  getting-started/  setup.md
  agent-lifecycle/  agent-hierarchy.md, approvals.md, persistence.md
  trust-boundary/   boundary.md, security.md
  integrations/     forge.md, matrix.md, github.md, knowledge.md
  networking/       gateway.md, network.md, snapshot-store.md
  scheduler/        jobq.md, coordinator.md, ci.md, observability.md
  process/          conventions.md, gotchas.md, pr-review-gate.md
  web-ui/           terminal-rendering.md (moved into the EXISTING dir,
                    per mara's correction to the original getting-started
                    guess -- it's UI implementation detail, not onboarding)

The physical layout now matches docs/README.md's own topical headers,
which already amounted to this taxonomy -- see the scoping comment on
the issue for the two findings that motivated this (a genuine
duplication between CLAUDE.md's old "Reading paths" list and
docs/README.md's grouped one, since drifted out of sync with each
other; and the flat layout not matching the grouping we already had).

Fixed every cross-reference this moved across the whole repo (~120
files: docs/ internal links at every depth, Rust doc comments, nix
module option docs, crate READMEs) -- verified two ways: a grep sweep
confirming zero remaining references to any old path, and a script
that resolves every markdown link in docs/**/*.md + CLAUDE.md +
README.md against the filesystem and reports anything that doesn't
exist (zero broken links).

Collapsed CLAUDE.md's "Reading paths" section (the duplicate) down to
a pointer at docs/README.md, now the single index. Rewrote
docs/README.md itself to use the new subdirectory paths and added the
one doc it was missing that CLAUDE.md's old copy had (pr-review-gate.md).

Classified all 22 docs/*.md files first via a haiku subagent (mara's
suggestion) on two axes -- proposed grouping and operator-vs-
implementation focus -- before finalizing the taxonomy; spot-checked
the report and found internal inconsistencies (its classification
table disagreed with its own summary section for a few files), so this
taxonomy is my original proposal + the one correction mara gave
directly, not a blind application of the subagent's table. The
operator-focus data it gathered is still useful for a follow-up
content pass (docs skewing 'mixed' rather than pure operator-facing),
not addressed in this PR -- structure only.

nix fmt clean, both pre-push lints clean.

docs/web-ui/terminal-rendering.md

@@ -0,0 +1,51 @@
+# Per-agent terminal: row taxonomy (as built)
+
+The per-agent web UI's live pane renders one row per `TermMsg`
+(`hive-agent/src/term_msg.rs`): `{icon?, level: debug|info|warn|error,
+summary, body?, body_format?: markdown|diff, coalesce_key?}`. Classification
+(icon, summary text, whether a tool call gets an expandable body) happens
+server-side, once — `term_msg.rs` + `stream_enrich.rs` — and is served
+identically by both `GET /api/events/history` and `GET /api/events/stream`
+as `TermEnvelope { ts, seq?, msgs: TermMsg[] }` frames
+(`hive-agent/src/web_ui/stream.rs`).
+
+The frontend renders a `TermMsg` close to as-is
+(`frontend/packages/agent/src/components/Row.tsx`): `level` picks the
+CSS colour (`terminal.css`'s `.live .level-*`), an empty `summary` +
+markdown `body` renders as a flat row with just the body (assistant
+text), and any other bodied row is an expandable `<details>`, opened by
+default according to the operator's expand-tool-output preference
+(`getExpandDetailsPref()`) — uniformly, no per-tool override. There's no
+separate client-side row model or classification step.
+
+## Layout
+
+Every row shares one prefix column via `padding-left` + negative
+`text-indent` on `.live .row`; an icon (when set) sits in a fixed-width
+`.row-glyph` cell so icons of different rendered widths still line up.
+`<details>` summaries reuse the same metrics, with the disclosure caret
+leading the summary text rather than the icon.
+
+## Levels
+
+| Level | Colour | Roughly |
+|---|---|---|
+| `debug` | muted | thinking, coalesced ticks, ambient harness chatter |
+| `info` | default fg | turn start/ok, assistant text, tool calls + results |
+| `warn` | amber, left rule | stderr, an unrecognised event shape, API retries |
+| `error` | red, left rule | turn failed, a tool result with `is_error: true` |
+
+Per-tool icon + summary formatting (what a `Read`/`Edit`/`send` call's row
+actually says) lives in `stream_enrich.rs`'s `fmt_tool_use()` family —
+read that when you need the specifics, this doc doesn't duplicate it.
+
+## Markdown
+
+`frontend/packages/agent/src/lib/markdown.ts`'s `renderMarkdown()` runs
+`marked.parse()` through DOMPurify into a `<div class="md">`. Applied to
+any `TermMsg` with `body_format: "markdown"`.
+
+## Dashboard side (not covered here)
+
+The main dashboard's message-flow pane is a different shape: broker
+messages render as `.msgrow` grid lines, not agent-terminal rows.