feat(tracker): add Gantt scheduling schema (startDate + IssueRelation)

Schema-only foundation for the upcoming Gantt-chart view in tracker.
No UI in this PR.

Changes:
- Issue.startDate: Timestamp | null (interface + IssueDraft + @Prop with @Index)
- Milestone.startDate: Timestamp | null (interface + @Prop, reusing the
  existing tracker.string.StartDate IntlString)
- New DependencyKind type ('finish-to-start' | 'start-to-start' |
  'finish-to-finish' | 'start-to-finish')
- New IssueRelation AttachedDoc class with kind: DependencyKind, signed
  lag: number — registered in models/tracker via TIssueRelation
- 7 new IntlString keys: IssueStartDate, GanttDependency,
  GanttDependency{FinishToStart,StartToStart,FinishToFinish,StartToFinish},
  GanttLag — all 13 locales updated
- Cross-plugin literal updates in importer + github sync to satisfy the new
  required Issue.startDate / Milestone.startDate fields:
  - packages/importer/src/importer/importer.ts: AttachedData<Issue> literal
  - services/github/pod-github/src/sync/issueBase.ts: 'startDate' added to
    GithubIssueData Omit list (github sync does not own scheduling)
  - services/github/pod-github/src/sync/issues.ts + pullrequests.ts:
    AttachedData<Issue|GithubPullRequest> literals

Out of scope (deferred to follow-up PRs):
- UI for Gantt view, drag/resize, dependency editor, critical path
- blockedBy → IssueRelation migration (ships atomically with the writer
  redirect in the dependency-UI PR)
- LinkIssues permission (tracker uses forbid-style permissions; needs
  maintainer discussion)
- Activity-feed wiring for IssueRelation (needs a producer to test against)
- IssueTemplate.startDate (template propagation semantics undecided)

Signed-off-by: Michael Uray <michaeluray@users.noreply.github.com>

services/github/pod-github/src/sync/issueBase.ts

@@ -69,6 +69,7 @@ WithMarkup<Issue>,
 | 'reports'
 | 'childInfo'
 | 'dueDate'
+| 'startDate'
 | 'kind'
 | 'reviews'
 | 'reviewThreads'

services/github/pod-github/src/sync/issues.ts

@@ -868,6 +868,7 @@ export class IssueSyncManager extends IssueSyncManagerBase implements DocSyncMan
       rank: calcRank(lastOne, undefined),
       comments: 0,
       subIssues: 0,
+      startDate: null,
       dueDate: null,
       parents: [],
       reportedTime: 0,

services/github/pod-github/src/sync/pullrequests.ts

@@ -1169,6 +1169,7 @@ export class PullRequestSyncManager extends IssueSyncManagerBase implements DocS
       rank: calcRank(lastOne, undefined),
       comments: 0,
       subIssues: 0,
+      startDate: null,
       dueDate: null,
       parents: [],
       reportedTime: 0,