docs: add Phase 1 execution plan (#2518)

* docs: add Phase 1 execution plan (design-documents)

Detailed, reviewed execution plan for Phase 1 (Developer Experience Core):
prioritization, v0.16–v0.19 release sequencing, per-workstream acceptance
criteria, edge-case mitigation, and a UX-cohesion through-line (CLI/UI/MCP/
library as four renderings of one operation set).

Revised against two independent codebase-grounded reviews. Key locked decisions:
SIGTERM graceful drain ships as a v0.15.1 correctness patch (live invariant-7
gap); conduit init is reconciled to a single 5-min-wow entry point; the UI
rebuild is a greenfield React app in a new conduit-ui repo. Advances ROADMAP
Phase 0 ("publish roadmap + board") and scopes Phase 1.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs: fix markdownlint in Phase 1 plan (lists, headings, table style)

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: devarismeroxa