Rename the binary to ellipsis and link el next to it

[hbrooks]

What

• The binary is ellipsis. el is a relative symlink the installer creates next to it, so both names run the same file.
• install.sh: installs <dir>/ellipsis, links <dir>/el, and leaves an existing el alone if it is not our link. The startup-file marker is now "Ellipsis CLI installer".
• ellipsis update: unchanged in behavior. The link points at the name, and the running executable resolves to the real file even when invoked as el.
• ellipsis uninstall: also removes el, only when it is a symlink to our binary.
• Release assets become ellipsis-<target>.tar.gz; the compile outfile, .gitignore, and the CI smoke test follow. CI now also runs el --version and uninstalls through el.
• Every command example in help text, README, both skills, the docs, and the smoke scripts uses ellipsis. Prose about coding agents is untouched.
• Help keeps one spelling: el --help prints Usage: ellipsis ....

Notes

• No hidden alias for the old agent name, by design.
• ALIAS_NAME, BINARY_NAME, and PATH_MARKER in src/lib/install.ts are checked against install.sh by tests.

Testing

• bun run typecheck, bun run test (239 tests).
• Installed from a local release layout into a temp dir: el links to ellipsis, el --version works, el uninstall removes both files and the PATH line.
• Installed for real on macOS arm64; ellipsis and el both resolve from a fresh login shell and el auth status reaches prod.