feat(workflows): support project-specific extra targets

[ArcanePivot]

Summary

• add an optional extra_targets_json input to the reusable CI, nightly, and release workflows
• append project-specific targets to the shared defaults instead of requiring callers to copy the full matrix
• validate required target fields and reject duplicate target names
• support an optional per-target build_args override
• document a MIPS little-endian musl example

Motivation

Projects occasionally need one extra architecture without changing the defaults for every Null-family repository. Replacing targets_json works, but it duplicates the shared matrix in the caller and can drift as nullbuilder evolves.

This is motivated by a verified NullClaw MIPS target for an HC5861 router running OpenWrt 23.05.5 (mipsel_24kc, MIPS32r2 soft-float). Keeping it project-specific avoids forcing unrelated Null projects to support MIPS.

Validation

• actionlint v1.7.12 passes
• git diff --check passes
• local jq cases cover empty extras, MIPS append, duplicate names, and invalid per-target arguments
• NullClaw d8a802f builds with Zig 0.16.0 using:

zig build -Doptimize=ReleaseSmall \
  -Dtarget=mipsel-linux-musleabi \
  -Dversion=pr-mips-validation \
  -Dembedded_wasm3=false

The resulting 7,044,980-byte static MIPS32r2 binary reports the embedded version and starts an isolated gateway on the physical HC5861. /health returns {"status":"ok"} and idle RSS is about 4.7 MB. The production agent was not modified or restarted.

Compatibility

The new input defaults to [], so existing callers keep the same matrices and build arguments. build_args remains optional on every target.

Embedded wasm3 is disabled only for this proposed MIPS target because its C build currently crashes the Zig/Clang frontend for mipsel-linux-musleabi; NullClaw default channels and SQLite remain enabled.

After merge, the reusable workflow v1 branch/reference will need to be updated before a follow-up NullClaw PR can consume the new input.