docs: add Troubleshooting FAQ and Dependabot config

[TheStreamCode]

P1 contributor-funnel polish per repo review (README Dim 2, Automation/Security Dims 4/7).

Changes

• README Troubleshooting section (+ nav link): three failure modes, each grounded in-repo:
  • Missing CHUTES_API_KEY — first-launch credential, process env (no auto .env loading), inspect --json check (docs/configuration.md).
  • Model-pool routing errors — saved default pool vs CHUTES_ROUTING_POOL inline pool, fallback chain, CHUTES_STRICT_MODEL, CHUTES_ROUTING_STRATEGY (docs/configuration.md Native routing).
  • Windows console glyph degradation — legacy ConHost ASCII/CP437 fallbacks, use Windows Terminal/VS Code; notes CI forcing modern glyphs via CHUTES_BUILD_FORCE_LEGACY_CONSOLE=0 (glyphs.rs, ci.yml).
• .github/dependabot.yml (new): cargo + npm + github-actions ecosystems, weekly grouped schedules.

Non-goals (untouched)

• No release trigger change (P2 tag-triggered switch out of scope); no awesome-list PR.

Verification

• npm test: 16/16 pass.
• actionlint: not installed locally — skipped (no workflow files touched).
• cargo fmt --check: N/A — no Rust files touched (README + YAML only).
• Dependabot YAML presence/sanity checked via node read.