chore(comments): drop epic, sprint and review-round ids from the code

The code cited its plan instead of saying why: `E28 review J4`, `(R6 S3b)`, `S1 justified…`,
`deep-review round 5`. That plan lives in the private ops repo, so for every reader of this one the
citation was noise, and a sentence whose subject was the plan step ("round 2 showed…") hid its
lesson behind an id. Each one is gone and the "why" kept: a bare citation is deleted, a sentence
carrying a lesson is reworded around it ("a later attempt showed…").

Measured (`scripts/audit_metrics.py`, product code): 652 lines in 270 files carried an epic code by
the definition the next commit holds (507 / 241 by the old, two-digit-only one) -> 0. Across all
code, tests, scripts and configs: ~1100 epic-code lines plus ~320 sprint/round ids.

Behaviour unchanged, checked mechanically: every changed .py file has the same AST once docstrings
are blanked; css/sh/toml/yml/json files are identical once comments are stripped; the only
non-comment changes in TS/JS are test titles, one description string in a contract table, and five
ESLint messages, whose text is all the gates snapshot diff holds. Not touched: Markdown (docs and
the changelog keep their history), Alembic migrations, TV episode notation (`S2 · E5`), DESIGN.md
ids (`C1`, `B4`, `S3h`), tracker ids (`SEC-16`, `DX-17`).

One stale claim fixed on the way: test_hls_reaper's docstring said the player checkpoints "every
10 s whether or not it is playing", which plex/stream.py calls wrong; it now says what the code does.

frontend/src/lib/api/diagnostics.ts

@@ -1,4 +1,4 @@
-// The user's diagnostics level (E22 S5) and the browser log it lets out. Only touches core's req and
+// The user's diagnostics level and the browser log it lets out. Only touches core's req and
 // beacon. The streaming itself — what is sent, when, and when it stops — is `lib/diagnostics.ts`.
 import type { LogEvent } from "../log";
 import type { ServerLogRow } from "./admin";
@@ -37,7 +37,7 @@ export const diagnosticsApi = {
   sendLogEventsOnUnload: (events: LogEvent[]): void => beacon(DIAGNOSTICS_EVENTS_PATH, { events }),
 };
 
-// --- debug sessions (E16) ----------------------------------------------------------------------
+// --- debug sessions ----------------------------------------------------------------------
 
 /** The two levels a recording can run at — it always records more than the base. */
 export type DebugSessionLevel = "DEBUG" | "TRACE";