Add detailed checkpoint push progress during pre-push

[SnowingFox]

Problem / intent

Closes #1683.

When git push runs Entire's pre-push hook, checkpoint sync can sit there for a long time — fetch, cherry-pick rebase onto remote tip, retry push, OPF, etc. Previously users only saw:

[entire] Pushing entire/checkpoints/v1 to origin...
.....
 done

No phase labels, no commit/session context, no transfer stats. A slow push feels stuck even when it's working.

This PR aligns Entire's stderr UX with Story CLI (story/apps/cli): structured, phased progress on stderr so users always know what phase is running. Display-only — failures still degrade gracefully (user's main push continues) except existing OPF privacy aborts.

Before → after

Before	After
Dot spinner (.....)	Named phases with elapsed time
No pre-push context	Session tree: how many commits/sessions are pending
Opaque sync/rebase	fetching…, remote is N ahead, rebasing i/M…
Git transfer invisible	counting / compressing / writing from git --progress

Example output (happy path + sync recovery)

[entire] Checkpoint push: 12 commits, 3 sessions
         ├─ sess-2026-06-12-abc123  3 checkpoints  (12:30 ~ 14:05)
         └─ sess-2026-06-11-def456  2 checkpoints  (yesterday)
[entire] Pushing entire/checkpoints/v1 to origin...
         counting objects: 47
         compressing: 38/38
         writing: 42 objects, 156.23 KiB, 312.00 KiB/s... done
         done (3s)
         conflict (2s)
[entire] Syncing with remote...
         fetching from origin... done (1s)
         remote is 3 commits ahead, rebasing 2 local commits...
         rebasing 1/2... (0s)
         rebasing 2/2... (1s)
         rebasing done (2s)
[entire] Pushing entire/checkpoints/v1 to origin...
         done (1s)

Notes on the labels:

• Checkpoint push: N commits, M sessions — pre-push summary from unpushed entire/checkpoints/v1 commits, grouped by Entire-Session trailer.
• counting / compressing / writing — parsed from git push --progress stderr (done state only, avoids spam).
• conflict (Xs) — generic marker that the first push failed and we're entering sync/recovery (not a merge-conflict UI; cherry-pick is conflict-free by design).
• rebasing i/M — in-memory cherry-pick onto remote tip, not git rebase.

What changed

New: push_progress.go + tests

Pure parsing/formatting (no I/O), TDD'd against Story's reference:

• parsePushSummaryFromLog — group checkpoint commits by session
• formatSessionTree — tree display (max 5 sessions, new branch label)
• parseGitProgressLine / displayGitProgress — git transfer stderr → indented lines
• Styled via interactive.ShouldStyle (respects NO_COLOR)

push_common.go

• Replaced startProgressDots with phased output + elapsed seconds
• doPushRef: push → on failure conflict + Syncing with remote... → retry push
• fetchAndRebaseRefCommon: fetch done line, remote-ahead summary, per-commit rebasing i/M, rebasing done

manual_commit_push.go

• printPushSummary() before push loop — silent on any failure, never blocks push

checkpoint/remote/git.go

• Add --progress to git push; capture stdout (porcelain) and stderr (progress) separately

metadata_reconcile.go

• cherryPickOnto accepts optional progress callback for rebasing i/M lines

Out of scope

• OPF progress lines (unchanged; already has its own notices in redact/batch.go)
• Real merge-conflict resolution UI (not applicable to checkpoint cherry-pick path)

Test plan

• mise run check (unit + integration + Vogon canary)
• push_progress_test.go — parsers/formatters
• push_common_test.go — already up-to-date / done output updated
• Manual: git push with unpushed v1 commits → session tree + transfer lines
• Manual: two clones, diverge entire/checkpoints/v1 → full sync sequence above