feat(providers): add Pi coding agent

[StiensWout]

Problem

Pi users depend on their existing models, auth, extensions, skills, context files, and native session history. T3 Code did not have a Pi provider, and a shallow CLI wrapper would lose the customization and session behavior that make Pi useful.

Solution

This adds Pi as an Early Access provider on Orchestrator V2:

• T3 speaks Pi's stdio JSONL RPC protocol through the user's own installation and discovers models, thinking levels, commands, skills, and auth from Pi itself.
• T3 threads use Pi's native session files for resume, current-branch fork, rollback, steering, and session naming.
• Extension dialogs, notifications, status rows, MCP tools, and resumable subagents project into the existing T3 lifecycle surfaces.
• The launcher replaces only Pi's conflicting official subagent extension, deduplicates explicit extension arguments, and restores trusted user and project extensions.
• Pi's session statistics feed the shared composer context meter. The ring tracks the active thread's context window while the popover exposes cumulative tokens processed across that Pi session.
• Turn settlement, retry and compaction recovery, child cancellation, process teardown, and fork rollback paths fail closed without leaking or corrupting the active native session.

The MCP bridge, resumable subagent work, and per-model thinking support originated in @mwolson's stacked contribution and retain that authorship.

Related work

Closes #397.
Closes #402.
Addresses #6685.

This supersedes the Pi implementations in #2211, #2748, #2800, #2812, #2831, #2856, #3818, #3947, #4355, #4445, #5688, #5882, and #6319.

This incorporates StiensWout#34 by @mwolson.

Validation

• 33 focused Pi tests, reduced from 48 by removing duplicate helper and branch matrices while strengthening observable lifecycle coverage.
• Targeted contracts, server, and web typechecks.
• Targeted changed-file lint and formatting.
• Both generated Pi extensions parsed independently as TypeScript.
• Live validation against Pi 0.84.2, including provider discovery, multi-turn context usage, MCP, extension UI, native subagents, steering, rollback, and provider handoff.
• Mobile parity audited for the shared Codex and Claude surfaces; Pi-specific mobile behavior is intentionally limited to provider identity and model selection.

Proof

Provider discovery

End-to-end Pi turn

Context meter persists across turns

The active context stays at 21k/272k while Pi's cumulative session total rises from 90k to 111k after the next completed turn.

Native resumable subagent

Native steering

Checkpoint and Pi session-tree rollback

Stacked on #2829 (t3code/codex-turn-mapping) until that branch lands.

Built by GPT-5.6 Sol in T3 Code through Codex.

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