modify: only require submit when changes affect PRs

[skarim]

Previously, gh stack modify always transitioned to PhasePendingSubmit
after completing on any stack with a remote ID (s.ID != ""). This blocked
the user from running another modify until they ran gh stack submit,
even when the modifications only touched local branches without PRs.

This was overly restrictive. If a user is working at the top of their stack
with branches that haven't been pushed or had PRs created yet, restructuring
those branches is a purely local operation — there is no remote state to
reconcile, and no reason to force a submit before allowing further modifies.

What changed

The condition for entering PhasePendingSubmit is now
s.ID != "" && affectsPRs instead of just s.ID != "".

A new affectsPRs flag is tracked throughout the apply process. It is set
to true when any of the following occurs:

• A renamed branch has a PullRequest ref
• A folded branch (source or target) has a PullRequest ref
• A dropped branch has a PullRequest ref
• A rebased branch (during cascading rebase) has a PullRequest ref

If none of these conditions are met, the modify state file is cleared
immediately — no pending-submit lock, no "run gh stack submit" prompt.

Changes by file

internal/modify/state.go

• Added AffectsPRs bool field to StateFile. This persists the flag
  across conflict boundaries so that ContinueApply knows whether
  actions applied before the conflict already affected PR branches.

internal/modify/apply.go

• ApplyPlan: tracks affectsPRs through each step (rename, fold, drop,
  rebase). Saves the flag into conflict state when a conflict occurs.
  Uses s.ID != "" && affectsPRs for the pending-submit decision.
• ContinueApply: initializes affectsPRs from the saved state file,
  then checks the conflict branch and remaining branches for PRs during
  the cascading rebase. Uses the same combined condition.
• Both functions set result.NeedsSubmit / show the "run submit" message
  only when the flag is true.

internal/tui/modifyview/types.go

• Added NeedsSubmit bool to ApplyResult so the caller can use it
  for the success message.

cmd/modify.go

• printModifySuccess now takes its cue from result.NeedsSubmit
  instead of s.ID != "". The "run gh stack submit" hint is only
  shown when PR branches were actually affected.

internal/modify/apply_test.go

• Updated TestApplyPlan_PendingSubmitForRemoteStack to use branches
  with PRs and trigger an actual rebase, validating the pending-submit
  path correctly.
• Added TestApplyPlan_ClearsStateForRemoteStackWithNoPRBranches:
  remote stack where no branches have PRs → state is cleared.
• Added TestApplyPlan_PendingSubmitOnlyWhenPRBranchesAffected:
  remote stack with a mix of PR and non-PR branches, only the non-PR
  branch is renamed → state is cleared, NeedsSubmit is false.

Behavior summary

Scenario	Before	After
Modify on local stack (no remote ID)	State cleared	State cleared (unchanged)
Modify on remote stack, PR branches affected	PhasePendingSubmit	PhasePendingSubmit (unchanged)
Modify on remote stack, only local branches affected	PhasePendingSubmit ❌	State cleared ✅

The CheckStateGuard function (used by add, push, sync, unstack,
rebase) already did not block on PhasePendingSubmit, so those commands
are unaffected by this change.

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