feat: context engine plugins, scaffolding, and pricing fixes (#6)

* feat: add 4 context engine plugins

- topic-memory: keyword clustering for topic-aware memory recall
- episodic-memory: conversation segmentation and cross-session recall
- user-profile: persistent user profiling from conversation patterns
- context-decay: time-based memory decay with reinforcement dynamics

All plugins use the ingest/after_turn hook protocol with stdin/stdout JSON.

* chore: add plugin scaffolding, update docs and templates

- Add plugin.toml template with {{NAME}} placeholder
- Add new-plugin Makefile target with hooks/ scaffolding
- Update plugins/README.md with all 10 plugins
- Update README.md stats (10 plugins, 220+ models)
- Add Plugin checkbox and checklist to PR template
- Add Plugin to issue template content type dropdown
- Fix CONTRIBUTING.md: last_verified is recommended, not required

* fix: correct model pricing and remove deprecated entries

- openrouter/gemma-2-9b-it: fix pricing from 0.0 to 0.03/0.09 per M tokens
  (free variant correctly stays at 0.0)
- github-copilot: remove deprecated copilot/gpt-4 model entry
  (GPT-4 retired in favor of GPT-4o for Copilot)

* docs: annotate kimi-coding as membership-gated

Kimi Code CLI uses quota-based membership model (not per-token billing).
Free tier has limited weekly requests; underlying model is K2.5.
Pricing kept at 0.0 consistent with other subscription providers
(chatgpt, github-copilot) but with explanatory comments.

* style: fix trailing newline in github-copilot.toml

* fix: correct Moonshot/Kimi model pricing from official sources

All 5 models had incorrect pricing:
- moonshot-v1-8k: 0.10/0.10 → 0.20/2.00
- moonshot-v1-32k: 0.30/0.30 → 1.00/3.00
- moonshot-v1-128k: 0.80/0.80 → 2.00/5.00
- kimi-k2: 2.00/8.00 → 0.60/2.50
- kimi-k2.5: 2.00/8.00 → 0.45/2.20

Sources: platform.moonshot.ai/docs/pricing/chat, costgoat.com, getmaxim.ai

* feat: add MiniMax M2.7 and M2.7-highspeed models

Released 2026-03-18, MiniMax's latest flagship text model.
10B activated params, 200K context, 128K output, tool use, streaming.
Pricing: $0.30/$1.20 per M tokens (input/output).

Added to both international (minimax.io) and China (minimaxi.com) providers.

providers/minimax-cn.toml

@@ -1,5 +1,5 @@
 # MiniMax (China) — https://minimaxi.com
-# Models: 6 (same models as international, different endpoint)
+# Models: 8 (same models as international, different endpoint)
 
 [provider]
 id = "minimax-cn"
@@ -8,6 +8,32 @@ api_key_env = "MINIMAX_CN_API_KEY"
 base_url = "https://api.minimaxi.com/v1"
 key_required = true
 
+[[models]]
+id = "MiniMax-M2.7"
+display_name = "MiniMax M2.7"
+tier = "frontier"
+context_window = 204800
+max_output_tokens = 131072
+input_cost_per_m = 0.30
+output_cost_per_m = 1.20
+supports_tools = true
+supports_vision = false
+supports_streaming = true
+aliases = []
+
+[[models]]
+id = "MiniMax-M2.7-highspeed"
+display_name = "MiniMax M2.7 Highspeed"
+tier = "smart"
+context_window = 204800
+max_output_tokens = 131072
+input_cost_per_m = 0.30
+output_cost_per_m = 1.20
+supports_tools = true
+supports_vision = false
+supports_streaming = true
+aliases = []
+
 [[models]]
 id = "minimax-text-01"
 display_name = "MiniMax Text 01"