/office-hours always reports SESSION_COUNT: 0, welcome_back tier is unreachable

# `/office-hours` always reports `SESSION_COUNT: 0`, welcome_back tier is unreachable

Every `/office-hours` run on a fresh `$HOME` returns `TIER: introduction` and fires the full closing pitch. Doesn't matter how many times the user has run it. The `welcome_back` tier in `bin/gstack-developer-profile:165-169` (which exists specifically to skip the closing pitch for returning users) is never reached on fresh-`$HOME` installs.

The v1.0.0.0 migration moved the read path to `~/.gstack/developer-profile.json`, but the writer in `office-hours/SKILL.md` (lines 1560 and 2022 as of 2026-05-24) still appends to `~/.gstack/builder-profile.jsonl`. On first read, `ensure_profile()` creates a stub `developer-profile.json` with `sessions: []`. Later writes go to a file the reader never re-reads. `SESSION_COUNT` stays 0 forever.

Live since `0a803f9` / PR #1039 / v1.0.0.0 (2026-04-18). 5 weeks on every fresh-`$HOME` user.

## Repro

Fresh `$HOME`, run `/office-hours` twice (or check `~/.gstack/` between runs):

```
05:26 ~/.gstack/developer-profile.json created (sessions: [], empty stub)
05:53 ~/.gstack/builder-profile.jsonl appended (today's session)
```

27-minute gap inside one skill run. Reader saw the empty stub at preamble; writer appended to the legacy file at completion. Second `/office-hours` run re-reads the stub, still sees `sessions: []`, still picks the introduction tier.

## Fix

Either (a) move the writer to `developer-profile.json`, or (b) move the reader back to `builder-profile.jsonl`. (a) is the direction v1.0.0.0 was going; (b) is a revert. Option (a) needs a small subcommand on `gstack-developer-profile` that read-modify-writes the `sessions[]` array. Whoever owns this should also decide on stranded entries in `builder-profile.jsonl` from the broken-writer period (leave them; loss is aesthetic).

Filing as a regression report, not a PR. Happy to send a PR if useful; flag here first since the choice of (a) vs (b) is yours.