feat(core): external deployment id wire contract (#4662)

An external deployment id is an opaque, caller-chosen name for a release
- a commit SHA, a CI run id, a release tag. This adds the shared
contract that both halves of the feature read, and nothing else: no
deploy writes one yet and no trigger sends one.

ExternalDeploymentId is defined once and reused by
InitializeDeploymentRequestBody.externalId and
TriggerTaskRequestBody.options.externalDeploymentId, so a value accepted
by one half can never be rejected by the other. A value that is blank
once trimmed is treated as absent rather than rejected, so an unset CI
variable expanding to an empty string is not a 400. The 128 character
limit fits a SHA-256 commit hash with room for composite ids, and
EXTERNAL_DEPLOYMENT_ID_MAX_LENGTH is the single source of truth that the
request schemas and the CLI both read.

RunAnnotations.externalDeploymentId records the request, not the
outcome: lockedToVersionId and taskVersion are overwritten when a run
locks, whereas this stays true forever, and it can carry the pin for a
run parked before its deployment exists.

Also lands the runtime discovery helpers as pure functions over an
environment reader: the explicit TRIGGER_EXTERNAL_DEPLOYMENT_ID
variable, the platform and CI commit-SHA table, and the
TRIGGER_AUTOMATIC_SKEW_VERSION_PROTECTION gate. Nothing calls them yet.

refs TRI-13000