Add Autoclave (Autokey) cipher codec

[Copilot]

Checklist

• No unrelated changes
• Codec is new (not already implemented)
• Tests included (if cannot be automated with tests/test_generated)
• Documentation (included in the right page in docs/pages/enc)

Description

The Autoclave (Autokey) cipher is a Vigenère variant where the keystream is initial_key + plaintext, ensuring the key is always at least as long as the message. Decoding is iterative: each decoded character extends the working keystream.

Implementation (src/codext/crypto/autoclave.py):

• Accepts codec names matching autoclave-KEY or autokey-KEY (and *-cipher-* variants)
• Preserves non-alphabetic characters; case of input is preserved in output
• __examples__ includes a known test vector and round-trip cases; auto-generated tests via test_generated

import codext

codext.encode("ATTACKATDAWN", "autoclave-queenly")  # -> "QNXEPVYTWTWP"
codext.decode("QNXEPVYTWTWP", "autoclave-queenly")  # -> "ATTACKATDAWN"

codext.encode("Hello World", "autokey-secret")      # spaces/non-alpha passed through

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add new encoding: autoclave</issue_title>
<issue_description>### Description

Autoclave/Autokey encryption is a variant of key encryption, it has the particularity of modifying the key by concatenating the key and the message (key+message).

Reference

https://www.dcode.fr/autoclave-cipher</issue_description>

<agent_instructions>Please propose an implementation according to the guideline at docs/pages/howto.md.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Add new encoding: autoclave #31

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