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/useHoverFocusWithin.ts

@@ -63,7 +63,7 @@ export function useHoverFocusWithin() {
       // A dialog opened FROM inside the panel (the rail row's "Delete?" confirm) renders at the app
       // root, so neither the DOM nor the React tree sees it as inside — and collapsing the panel under
       // it unmounted the very row the dialog hands the focus back to, leaving a keyboard user on
-      // <body> after Cancel (measured in Chrome, E11 S2). Hold while the focus is in a dialog: when it
+      // <body> after Cancel (measured in Chrome). Hold while the focus is in a dialog: when it
       // closes, the focus either comes back in here (onFocus) or lands nowhere, which the
       // document-level net below clears.
       if (to?.closest('[aria-modal="true"]')) return;