docs: repoint dead development-stages.md links to v2 archive + milestone roadmap

[hanwencheng]

What & why

~47 markdown/doc links pointed at docs/spec/plans/development-stages.md, which was archived to docs/archived/development-stages-v2-2026-04.md (and docs/spec/plans/ no longer holds it). They were broken; this PR repoints every one by context.

Repointed by context

→ docs/archived/development-stages-v2-2026-04.md (historical 8-stage content — 42 refs): README.md, TODOS.md, threat-model-key-custody.md, broker-and-operator-dev-guide.md, heima-gaps-vs-desired-architecture.md, post-v0.1-future-work.md, execution-plan.md, all six wiki pages (Home, blockchain-tee-architecture, hosted-first, key-security, overview, serve-and-audit), and the CLI provision-error blob URLs in lib.rs (+ the mirrored literal in cli_tests.rs, kept in sync).

→ docs/spec/plans/milestones-roadmap.md (live "current roadmap" pointers — 3 refs): the README.md per-session protocol (aligned to CLAUDE.md wording), arch.md, and the wiki Home "Product/roadmap reviewer" row. arch.md points at the live roadmap rather than archived per CLAUDE.md's "arch.md never links into docs/archived/" rule — the roadmap itself links onward to archived.

Adjacent broken link also fixed: broker-and-operator-dev-guide.md ./plans/issue-101-ci-auto-deploy.md (never existed) → ../ci-setup.md (the real CI auto-deploy doc for issue #101 / PR #102).

Notable decisions

• Successor path: the roadmap lives at docs/spec/plans/milestones-roadmap.md (not docs/plan/, which holds pre-code plans) — confirmed by CLAUDE.md and the file's own "replaces the v1/v2 staged development-stages.md plan" line.
• Stage numbers preserved verbatim. Referrers use the pre-renumber scheme (Stage 8 = hardening, Stage 9 = Heima/Pattern-4); v2 archived renumbered these (8 = vault, 9 = hardening, 10 = Heima/Pattern-4). v2 archived contains Stage 0–10 so every link resolves; renumbering prose was out of scope.
• Two refs intentionally left: execution-plan.md:5,157 v2/plans/development-stages.md are project-life source paths (the 759-line original), not in-repo links.
• Dropped two stale line-anchors (:351-358 in key-security.md) that don't map onto the condensed archived file.

Verification

• Markdown broken-relative-link scan: 0 broken development-stages links; total repo broken-link count 191 → 190 (only the issue-101 fix — no new breakage).
• No old-path spec/plans/development-stages reference remains outside docs/archived/.
• cargo check -p agentkeys-cli → exit 0.

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