attest: Compute default for ENCODED_* from EPOCH, simpler NO_SIGN

[hodlinator]

Improves ergonomics of the asmap-attest script:

• Document required environment variables in help text
• Drop the requirement for ENCODED_* env vars as their default locations can be computed based off EPOCH now that the script and files (often) reside in the same repo.
• Drop requirement for SIGNER if NO_SIGN is set and only output SHA256SUMS in that case. (More subjective value than other commits).

Inspired by discussion at #50 (comment)

[jurraca]

ACK on the SIGNER/NO_SIGN change (103a6e5) but the ENCODED_* default paths change have tradeoffs that imo are problematic:

• the point of attestation is not to rely on the files here, but to provide an attestation that the user generated themselves, so that we can compare the attestations to the files in the tree. If the defaults are used, the user is attesting to what's already there. We want the user to provide their own encoded files, showing that they got a final result, and encoded it, and attested to those files.
• Ideally the user could provide the ASMAP_TXT as currently, and call out to the bitcoin/contrib/asmap/asmap-tool.py to encode the files on the fly, and generate the attestation from there. This is maybe a better approach, though it complicates the script a bit.
• On attesting to a new run, the default file paths do not exist, so they are required unless the files already exist.

$ env NO_SIGN=1 ASMAP_TXT=../kartograf/out/1780588800/final_result.txt EPOCH=1780588800 ./asmap-attest

ERR: The specified ENCODED_FILLED does not exist or is not a regular file:

    '2026/1780588800_asmap.dat'

[jurraca]

we should rm -rf $temp_sha256sums here too