fix(release): plumb RELEASE_TAG_PREFIX from .bootstrap.env into fastlane

[prakashrj]

Follow-up to #286, which shipped the knob incomplete. Found while adopting it downstream, not by re-reading the diff.

Two gaps

1. The setting was unreachable from .bootstrap.env. Bootstrap::Config is parsed from the file with no ENV merge, while Bootstrap::Version.tag_prefix reads ENV. So the field documented in .bootstrap.env.example did nothing — the one place a user would naturally set it.

2. asc_env did not propagate it. This is the worse one. Even with the env var set correctly, bin/ship.rb computes the tag in its own process and then launches fastlane with Bootstrap.asc_env(config) — which did not carry RELEASE_TAG_PREFIX. So:

ship.rb   → computes  app/v0.1.2+10
fastlane  → prefix is "v", tag does not start with it, `sub(/^v/,"")` no-ops
          → version = "app/v0.1.2+10"
          → every artifact name downstream built from that

A silent, wrong-output failure of exactly the kind this repo keeps running into: both processes succeed, and only the artifacts are wrong.

The fix

• Config#release_tag_prefix — process env → .bootstrap.env → "v". An explicitly empty value is honoured at both layers rather than falling through to "v", since empty is a legitimate choice (bare 1.0.0+5 tags).
• RELEASE_TAG_PREFIX added to Config::OPTIONAL.
• asc_env propagates it to the fastlane subprocess.
• bin/ship.rb and bin/compute-release-tag.rb publish the resolved value into ENV before computing, so the resolver and fastlane cannot disagree.

Verification

test/tag_prefix_test.rb grows 16 → 23, the new ones covering precisely this plumbing.

Mutation-verified:

mutation	assertions killed
drop the asc_env propagation (gap 2, as shipped)	1
make the accessor ignore .bootstrap.env (gap 1)	2

ci/check-shell.sh clean.