Add the MIT license (closes #14)

Adds LICENSE with the standard MIT text and "Copyright (c) 2026 sneak", the license sneak chose. The README now calls keyfunc MIT-licensed in its first paragraph, its License section points at LICENSE, and the license line leaves the TODO list.

Model: opus-5-5

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 sneak
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

@@ -1,11 +1,10 @@
 # keyfunc
 
-`keyfunc` is a Go command-line tool by [@sneak](https://sneak.berlin) — its
+`keyfunc` is an MIT-licensed Go command-line tool by
-license is not yet chosen
+[@sneak](https://sneak.berlin) that turns a BIP-39 mnemonic into SSH keys, age
-([#14](https://git.eeqj.de/sneak/keyfunc/issues/14)) — that turns a BIP-39
+identities and child mnemonics, each of which can be recreated from that
-mnemonic into SSH keys, age identities and child mnemonics, each of which can be
+mnemonic at any time. The same mnemonic, key type and index always give the same
-recreated from that mnemonic at any time. The same mnemonic, key type and index
+key.
-always give the same key.
 
 It uses the BIP-85 entropy deriver from `git.eeqj.de/sneak/secret/pkg/bip85` and
 takes the same steps as that repository's `agehd` package.
@@ -306,14 +305,11 @@ standard: most Makefile targets are thin shims over an executable in
 
 The open issues that stand between the tree and a 1.0 release:
 
-- [#14 Choose a license and add LICENSE](https://git.eeqj.de/sneak/keyfunc/issues/14)
 - [#15 Decide the Go module path before 1.0](https://git.eeqj.de/sneak/keyfunc/issues/15)
 
 ## License
 
-Not yet chosen. The license is the owner's decision, still open on the tracker
+MIT. The full text is in [`LICENSE`](LICENSE).
-([#14](https://git.eeqj.de/sneak/keyfunc/issues/14)); the `LICENSE` file is added
-when that issue is answered.
 
 ## Author