MCP slimdown — Move 2: CLI parity for 13 deprecation candidates

[mbachaud]

Parent spec: `docs/specs/2026-05-12-mcp-slimdown-plan.md` (PR #78).

Depends on / blocks: #79 (Move 1) — independent, but cleaner to land
identity unification first so deprecation warnings have a TOML-pointed
home for opt-in.

Goal

Build CLI peers for 13 of the 17 MCP tools we're deprecating so the
slimdown is non-breaking before tool removal.

Scope (4 new subcommand clusters)

`helix sessions {list, recent, register}`

• Replaces `helix_sessions_list`, `helix_session_recent`.
• `helix sessions list [--party X] [--status active|stale|all]`
• `helix sessions recent [--limit N] [--since UNIX_TS]`
• `helix sessions register --handle X --party Y [--host Z]` (the
  CLI equivalent of `POST /sessions/register` from MCP startup).
• Reuses `helix_context/bridge.py` — CLI becomes the third caller
  alongside MCP and the HTTP server.

`helix status` flag expansion

• `helix status --stats` → replaces `helix_stats`.
• `helix status --tokens` → replaces `helix_metrics_tokens`.
• `helix status --bridge` → replaces `helix_bridge_status`.
• All three coexist with existing `helix status` default output.

`helix diag` subcommand additions

• `helix diag gene ` → `helix_gene_get`
• `helix diag neighbors [--top-k N]` → `helix_neighbors`
• `helix diag splice ` → `helix_splice_preview`
• `helix diag resonance ` → `helix_resonance`
• `helix diag fingerprint ` → `helix_fingerprint`
• `helix diag refresh-plan ` → `helix_refresh_targets`
• `helix diag hitl [--since UNIX_TS] [--limit N]` → `helix_hitl_recent`

`helix consolidate`

• `helix consolidate [--source-ids X,Y,Z]` → `helix_consolidate`.
• Cold-start subcommand (single shot, exits).

Estimate

~3 days total; each subcommand is ~40 LOC of argparse + bridge call +
JSON/text formatter. Tests follow `tests/test_cli_*.py` pattern (one
test file per cluster).

Acceptance

• `helix --help` lists `sessions`, `consolidate` as first-class
  subcommands; `helix diag --help` lists 7 new operations.
• Each new subcommand has at least 3 tests (success + error + JSON).
• Existing MCP tools still work — this is additive, no MCP changes yet.
• Docs: `docs/clients/cli.md` gains 4 new sections.

Not in scope

• Removing or warning on MCP tools (that's Move 3).
• Touching `helix_announce` or `helix_hitl_emit` — deliberately
  CLI-incompatible per spec §"Non-goals" and §"HITL — split".

[mbachaud]

Partially shipped, scope changed. #82 (merged fa7561e) added 4 of the 13 candidates as CLI peers — helix packet, helix gene get/preview, helix neighbors, helix refresh-targets — but under different command shapes than this issue specified (top-level helix packet vs spec's helix diag packet), and crucially as additions, not as replacements for the MCP tools. The remaining 9 candidates can be filed as a fresh 'CLI scripting peers — additive surface' issue if/when prioritized; the demotion framing here doesn't carry forward.