style: clear standards-check lint debt (8x SC2312, 1x MD041)

[twistedmelonman]

Clears this repo's standards-check debt so the check passes. Wave 3 of the
fleet lint rollout (smartwatermelon/dev-env#100).

standards-check scans the whole repo on every PR, so these pre-existing
findings failed the check on any PR regardless of what it touched. Clearing
them in one shot is what makes the check green here.

What changed

shellcheck — 8x SC2312 in install.sh (lines 175, 277, 323, 393, 413,
520, 591, 603). Each is a command substitution whose exit status was already
discarded: $(...) inside [[ ]] or echo, and < <(...) process
substitution. Adding || true inside the substitution is SC2312's own
suggested alternative and adds no # shellcheck disable.

markdownlint — MD041 on LICENSE.md: added a # MIT License heading. The
text of the license is unchanged.

Comments at the four mapfile sites explaining why the guard is inert.

Why not the other SC2312 fix

The alternative — assign to a variable, then use it — was rejected because it
changes behavior twice over:

• mapfile -t x <<< "$out" yields a 1-element array containing "" on
  empty input, where < <(cmd) yields an empty array. read_manifest and
  parse_mirrors_file can legitimately produce no output.
• a bare assignment is subject to set -e, so a failing uname would exit
  instead of falling through to the || fatal guard at line 175.

On the "swallowed errors" reading

The local full-diff reviewer initially flagged these as a data-integrity
regression. They are not, and the second commit documents why in the source:

• the callees cannot fail — parse_mirrors_file and read_manifest each
  already end in || true; get_desired_mirrors either prints or calls fatal
• process substitution never propagates exit status under set -e regardless.
  Verified directly: set -euo pipefail; mapfile -t x < <(false; echo hi) does
  not abort.

The reviewer's underlying design concern is real but pre-existing, and is
filed separately as #33: read_manifest's inner || true conflates grep
exit 1 (no match) with exit 2 (unreadable file), so an unreadable manifest
reads as "nothing is managed". That one is load-bearing; these four are not.

Verification

• standards/run-standards.sh --repo . — exit 1 before, exit 0 after,
  all six linters clean. Known-bad control run first.
• bash -n install.sh passes; install.sh --help exits 0.
• The touched uname/readlink/date guards and the empty-input mapfile
  case smoke-tested to behave identically.
• shellcheck version parity confirmed: CI pins 0.11.0, local is 0.11.0.

https://claude.ai/code/session_01UaPoEix1iED8ENCZCa12jy

[claude]

All changes add `|| true` to command/process substitutions to satisfy ShellCheck SC2312, plus a cosmetic `# MIT License` heading fix (MD041). No behavioral changes: `|| true` in `$(uname -s || true)` only sets subshell exit code to 0 — stdout and the `== "Darwin"` comparison are unaffected. Process substitution exit codes don't propagate under `set -e` in Bash regardless, so the `mapfile` additions are correctly inert.

VERDICT: PASS