Startup banner and doctor never surface the resolved host name

Codex multi-host support (ADR-0011). Code under review is on branch feat/codex-support-m0, not main. ca-codex ships BETA; these are blocking-severity for the affected code but nothing is merged to the default branch yet.

Severity: med | Confidence: 0.8 | Group: no-host-marker-diagnostics

Where:

• core/pysrc/session-start.py:539-649
• core/pysrc/doctor.py:176-195

Evidence / impact:

• (observability-004) session-start.py's main() does host = hostapi.load_host() (line 541) and only ever reads two capability flags off it (host.has_statusline at line 556, host.plugin_root() at line 543) — host.name is read nowhere in the function. The printed '=== codeArbiter startup state ===' block (line 589 onward: stage, open questions, task board, provenance drift, update notice, briefing) contains no li
• (observability-004 impact) This branch's own commit history (e19778c, cited in inventory.md as evidence the silent-un-enforcement failure mode already happened once) was a hooks.json registration gap on Codex that went unnoticed until fixed post-hoc. Neither of the two places a maintainer is most likely to check 'is codeArbit

Recommendation:

Surface the resolved host.name in both session-start.py's startup banner and doctor.py's report, so a dormant Codex install is distinguishable from a healthy one at the place a maintainer looks.

Acceptance criteria:

• session-start and doctor output name the resolved host (claude/codex).