Summary

• Add --on <parent-ticket> flag to parsec start for creating dependent worktree chains
• Add parsec stack command to display the dependency graph as a tree
• Add parsec stack --sync to rebase the entire stack in topological order
• Stacked worktrees automatically use the parent's branch as the PR base when shipped

Changes

• src/worktree/lifecycle.rs: Added parent_ticket: Option<String> field to Workspace struct (backward-compatible via #[serde(default)])
• src/worktree/manager.rs: Modified create() to accept parent_ticket param; when --on is used, the parent's branch becomes the base
• src/cli/mod.rs: Added --on flag to Start variant, added Stack command variant with --sync flag, updated routing
• src/cli/commands.rs: Modified start() to pass on param, added stack() and stack_sync() command handlers
• src/output/mod.rs: Added print_stack() dispatcher
• src/output/human.rs: Added print_stack() with tree visualization (connectors, colors)
• src/output/json.rs: Added print_stack() JSON output
• README.md: Added parsec stack documentation section, updated parsec start with --on option, added stacked PRs to features list
• docs/index.html: Added Stacked PRs feature card

Test plan

• cargo build passes with no errors
• cargo clippy passes with no warnings
• cargo fmt --check passes
• Create a stack: parsec start T1, parsec start T2 --on T1, parsec start T3 --on T2
• parsec stack shows correct tree graph
• parsec stack --json outputs structured JSON
• parsec stack --sync rebases children onto parents in order
• parsec ship T2 creates PR with feature/T1 as base branch (not main)
• Existing state files without parent_ticket deserialize correctly (serde default)

Closes #29

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