fix(install): collapse the download meter to a single blue progress line

[plombeer31]

What

Installing the CLI currently prints curl's default transfer meter — a three-line table, twice
(once for the tarball, once for the checksum):

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 47.5M  100 47.5M    0     0  4372k      0  0:00:11  0:00:11 --:--:-- 5142k

This replaces it with a single line that stays on one line, in the project's own accent blue:

downloading atomic-agent  ████████████░░░░  76%  36.1 MB of 47.5 MB

How

• Both fetchers are silenced (curl -fsS -L, wget -q) and run in the background; the
  progress line is drawn from the growing output file against Content-Length, redrawn with
  \r + \033[K, and terminated by exactly one newline.
• The checksum file (99 bytes) downloads silently — no bar for a file that finishes instantly.
• Colour is #0b63f6, matching the badges in the README: 24-bit when COLORTERM advertises
  truecolor, 256-colour otherwise.

Degradation

Condition	Behaviour
Not a TTY (CI, | tee)	one plain line, no bar, no escape codes
NO_COLOR set, or TERM=dumb	bar kept, colour dropped
Non-UTF-8 locale	#/- instead of █/░
No Content-Length (chunked)	byte counter only, no percentage
wget instead of curl	same line, byte counter only

Bar width adapts to the terminal (24 / 16 / 8 blocks) so the line never wraps — a wrapped
line would smear across redraws. Width is read via stty size <&3, because inside the
command substitution that captures it fd 1 is the pipe, not the terminal, and tput cols
reports the terminfo default of 80 regardless of the real window.

Failure handling is unchanged

A failed transfer still aborts the install with curl's exit status; the progress line is
cleared first and the failing URL is printed. Checksum verification, the codesign check, and
the atomic binary/directory replacement are all untouched.

Testing

Verified on macOS arm64 against the real v0.2.1 release (47.5 MB):

• one line, 260 in-place redraws, zero newlines emitted during the download
• terminal widths 120 / 100 / 80 / 64 — bar 24 / 24 / 16 / 8, line always fits
• NO_COLOR=1, TERM=dumb, LANG=C, and piped-to-cat paths each checked
• full install verified end to end: checksum OK, 141 MB binary in place, binary runs
• failure path: ATOMIC_AGENT_VERSION=v0.0.0-does-not-exist exits non-zero, prints the URL,
  installs nothing

scripts/install.ps1 is untouched; the Windows meter is a separate change.